0001140361-26-026240 | 11-K | XBRL Viewer

Party-in-Interest Transactions	12 Months Ended
Dec. 31, 2025
NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]
Party-in-Interest Transactions [Abstract]
Party-in-Interest Transactions	5. Party‑in‑Interest Transactions Parties-in-interest are defined under Department of Labor regulations as any fiduciary of the Plan, any party rendering service to the Plan, the Employer, and certain others. Any transactions with parties-in-interest either fall outside the scope of, or are exempt from, ERISA’s prohibited transaction rules. The Plan's record keeper, Trustee, and Custodian, as well as the Company and Plan participants, and The Bonadio Group, the auditor of the Plan’s financial statements, are each a “party-in-interest" to the Plan as defined by ERISA. Purchases of Company stock amounted to $3,824,520 and $4,560,353 and sales amounted to $3,236,945 and $6,099,149 for the years ended December 31, 2025 and 2024, respectively. At December 31, 2025 and 2024, the number of shares of Company stock held in participants’ accounts totaled 724,330 and 737,569, respectively, with a fair value of $30,074,182 and $35,226,305, respectively. Notes receivable from participants totaling $3,492,310 and $3,450,393 at December 31, 2025 and 2024, respectively, qualify as party-in-interest transactions and are secured by balances in the respective participant accounts. Dividend income earned by the Plan includes dividend income from shares of NBT Bancorp Inc. common stock and amounted to $1,043,188 and $1,022,466 for the years ended December 31, 2025 and 2024, respectively.

Income Tax Status (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]	12 Months Ended
Dec. 31, 2025
Income Tax Status [Abstract]
IRS favorable opinion letter	true
IRS favorable opinion letter, date	Jun. 30, 2020
Tax-exempt qualified status	Qualified Plan [Member]

Accounting Policies, by Policy (Policies) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]	12 Months Ended
	Dec. 31, 2025
EBP, Accounting Policy [Line Items]
Basis of Presentation	Basis of Presentation The accompanying financial statements have been prepared on the accrual basis of accounting in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Amounts in prior year’s financial statements are reclassified, when necessary, to conform with current year’s presentation.
Investment Valuation and Income Recognition	Investment Valuation and Income Recognition The Plan’s investments are stated at fair value. The fair value of mutual funds and the common stock of the Sponsor are based on published market quotations. The fair value of the stable value fund is based on the calculated daily net asset values of the fund. Fair value is the price the Plan would receive to sell an asset or pay to transfer a liability in an orderly transaction with a market participant at the measurement date. In the absence of active markets for the identical assets and liabilities, such measurements involve developing assumptions based on market observable data and, in the absence of such data, internal information that is consistent with what market participants would use in a hypothetical transaction that occurs at the measurement date. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect our market assumptions. Preference is given to observable inputs. These two types of inputs create the following fair value hierarchy and a financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The three levels are defined as follows: Level 1 - Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities; Level 2 - Quoted prices for similar assets or liabilities in active markets, quoted prices in markets that are not active, or inputs that are observable, either directly or indirectly, for substantially the full term of the asset or liability; and Level 3 - Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (e.g., supported by little or no market activity). Investment transactions are recorded on a trade date basis. If a trade is open at the end of the year, a receivable for securities sold but not yet settled or a payable for securities purchased but not yet settled is reflected in the Statement of Net Assets Available for Benefits. Dividends are recorded on the ex-dividend date. Interest income is earned from settlement date and recognized on the accrual basis. The net appreciation in the fair value of investments consists of the realized gains or losses on the sales of investments and the net unrealized appreciation of investments. The methods described above may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Furthermore, although the Plan believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date.
Benefits Paid to Participants	Benefits Paid to Participants Benefit payments to participants are recorded when paid.
Notes Receivable from Participants	Notes Receivable from Participants Notes receivable from participants are carried at the unpaid principal balance plus interest.
Participant Withdrawals	Participant Withdrawals Participant withdrawals are recorded when paid.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of net assets available for plan benefits, disclosure of contingent assets and liabilities, and the reported amount of increases and decreases in net assets available for plan benefits. Actual results could differ from those estimates.
Risks and Uncertainties	Risks and Uncertainties The Plan invests in various types of investment securities. Investment securities are exposed to various risks, such as interest rate, market and credit risks. Due to the level of risk associated with certain investment securities, it is at least reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participant account balances and the amounts reported in the Statements of Net Assets Available for Plan Benefits.

- Definition

Disclosure of accounting policy for use of estimate in preparation of financial statement of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References + Details

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) - SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]

12 Months Ended

Dec. 31, 2025

USD ($)

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Asset held for investment

$ 414,701,477

Participant loans receivable [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Party-in-interest

true

^[1]

Description of investment

Interest rates [Member]

^[1]

Participant directed status

Participant-Directed Investments [Member]

^[1],[2]

Participant loans receivable

$ 3,492,310

^[1]

Investment, Identifier [Axis]: American Fund EUPAC Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

American Fund EUPAC Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

125,302

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 7,579,546

Investment, Identifier [Axis]: American Funds New Perspective Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

American Funds New Perspective Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

198,969

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 14,035,242

Investment, Identifier [Axis]: Cash [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Cash [Member]

Description of investment

Cash [Member]

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 31,469

Investment, Identifier [Axis]: Columbia Dividend Income Fund S [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Columbia Dividend Income Fund S [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

376,740

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 13,660,606

Investment, Identifier [Axis]: DFA Emerging Markets Portfolio Institutional [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

DFA Emerging Markets Portfolio Institutional [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

37,254

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 1,383,225

Investment, Identifier [Axis]: DFA US Sustainability Core 1 Portfolio [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

DFA US Sustainability Core 1 Portfolio [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

20,041

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 1,049,761

Investment, Identifier [Axis]: Dodge & Cox Income Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Dodge & Cox Income Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

740,667

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 9,517,569

Investment, Identifier [Axis]: Federated Hermes Capital Preservation Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Federated Hermes Capital Preservation Fund [Member]

Description of investment

Stable Value Fund [Member]

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 17,933,465

Investment, Identifier [Axis]: Fidelity 500 Index Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Fidelity 500 Index Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

193,452

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 45,987,495

Investment, Identifier [Axis]: Fidelity Inflation Protected Bond Index Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Fidelity Inflation Protected Bond Index Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

76,406

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 696,823

Investment, Identifier [Axis]: NBT Bancorp Inc. [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Party-in-interest

true

^[1]

Identity of issuer, borrower, lessor, or similar party

NBT Bancorp Inc. [Member]

^[1]

Description of investment

Common Stock of NBT Bancorp Inc. [Member]

^[1]

Number of shares (in Shares) | shares

724,330

^[1]

Participant directed status

Participant-Directed Investments [Member]

^[1],[2]

Current value

$ 30,074,182

^[1]

Investment, Identifier [Axis]: Schwab Value Advantage Money Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Party-in-interest

true

^[1]

Identity of issuer, borrower, lessor, or similar party

Schwab Value Advantage Money Fund [Member]

^[1]

Description of investment

Money Market Funds [Member]

^[1]

Participant directed status

Participant-Directed Investments [Member]

^[1],[2]

Current value

$ 519

^[1]

Investment, Identifier [Axis]: T-Rowe Price Dividend Growth Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Dividend Growth Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

139,291

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 11,364,716

Investment, Identifier [Axis]: T-Rowe Price Growth Stock Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Growth Stock Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

400,662

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 43,087,176

Investment, Identifier [Axis]: T-Rowe Price Mid Cap-Growth Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Mid Cap-Growth Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

119,864

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 11,655,580

Investment, Identifier [Axis]: T-Rowe Price Retirement 2010 Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement 2010 Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

84,112

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 1,345,785

Investment, Identifier [Axis]: T-Rowe Price Retirement 2020 Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement 2020 Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

899,776

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 17,761,583

Investment, Identifier [Axis]: T-Rowe Price Retirement 2030 Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement 2030 Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

1,997,972

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 55,443,730

Investment, Identifier [Axis]: T-Rowe Price Retirement 2040 Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement 2040 Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

1,359,620

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 46,770,911

Investment, Identifier [Axis]: T-Rowe Price Retirement 2050 Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement 2050 Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

1,606,807

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 34,771,297

Investment, Identifier [Axis]: T-Rowe Price Retirement 2060 Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement 2060 Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

486,037

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 9,229,852

Investment, Identifier [Axis]: T-Rowe Price Retirement Balance Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Retirement Balance Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

110,543

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 1,548,701

Investment, Identifier [Axis]: T-Rowe Price Small-Capital Value Fund [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

T-Rowe Price Small-Capital Value Fund [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

174,660

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 8,951,336

Investment, Identifier [Axis]: Vanguard Balanced Index Fund Institutional [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Vanguard Balanced Index Fund Institutional [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

137,779

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 7,138,315

Investment, Identifier [Axis]: Vanguard Intermediate US Treasury [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Vanguard Intermediate US Treasury [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

393,101

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 3,954,595

Investment, Identifier [Axis]: Vanguard Mid-Cap Index Fund Institutional [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Vanguard Mid-Cap Index Fund Institutional [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

179,727

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 14,266,742

Investment, Identifier [Axis]: Vanguard Short Term Bond Index Fund Admiral [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Identity of issuer, borrower, lessor, or similar party

Vanguard Short Term Bond Index Fund Admiral [Member]

Description of investment

Mutual Funds [Member]

Number of shares (in Shares) | shares

190,420

Participant directed status

Participant-Directed Investments [Member]

^[2]

Current value

$ 1,968,946

Minimum [Member] | Participant loans receivable [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Participant loans receivable, Interest rate

4.25%

^[1]

Maximum [Member] | Participant loans receivable [Member]

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]

Participant loans receivable, Interest rate

10.00%

^[1]

[1]	Party-in-interest.
[2]	Cost omitted for these participant directed investments.

Description of Plan (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]	12 Months Ended
	Dec. 31, 2025 USD ($) Age Hours	Dec. 31, 2024 USD ($)
Description of Plan [Abstract]
Plan agreement description	true
Employees hours of service completed to be eligible to participate in the Plan (in Hours) \| Hours	1,000
Employment period required for eligibility to participate in plan	30 days
Percentage automatically withheld from employee compensation and contributed to the Plan	6.00%
Company contributions	$ 6,591,657	$ 7,088,166
Minimum amount participants may borrow from their vested accounts	1,000
Maximum amount participants may borrow from their vested accounts	$ 50,000
Maximum percentage participants may borrow from their vested accounts	50.00%
Notes receivable from participants, Spread on Prime Rate	1.00%
Early retirement age	55 years
Early retirement minimum service period	5 years
Forfeiture from non-vested account balances	$ 149,133	145,368
Forfeitures used to reduce employer contributions	127,451	141,313
Forfeiture from account balances	$ 66,394	44,712
Participant’s Contribution Eligible for 100% Matching [Member]
Description of Plan [Abstract]
Company's matching contribution percentage	100.00%
Participant’s compensation percentage eligible for matching	1.00%
Participant’s Contribution From 1% to 5% Eligible for 50% Matching [Member]
Description of Plan [Abstract]
Company's matching contribution percentage	50.00%
Participant’s compensation percentage eligible for matching	5.00%
Company Matching [Member]
Description of Plan [Abstract]
Company contributions	$ 5,916,604	5,100,193
Discretionary Contributions [Member]
Description of Plan [Abstract]
Employees hours of service completed to be eligible to participate in the Plan (in Hours) \| Hours	1,000
Company contributions	$ 0	1,422,473
Employee benefit plan participants contributions vesting period	5 years
Worker Retirement Accumulation Plan [Member]
Description of Plan [Abstract]
Company contributions	$ 675,053	$ 565,500
Participants' Contributions and Net Investment Income of Loss [Member]
Description of Plan [Abstract]
Participants' contributions vesting percentage	100.00%
Safe Harbor Employer Matching Contributions [Member]
Description of Plan [Abstract]
Participants' contributions vesting percentage	100.00%
Employee vesting period	2 years
Termination Due to Death, Retirement or Permanent Disability [Member]
Description of Plan [Abstract]
Participants' contributions vesting percentage	100.00%
Loans, Other Than Loans for Purchase of Primary Residence [Member]
Description of Plan [Abstract]
Notes receivable from participants, repayment term	5 years
Loans for Purchase of Primary Residence [Member]
Description of Plan [Abstract]
Notes receivable from participants, repayment term	15 years
Minimum [Member]
Description of Plan [Abstract]
Employees age limit to be eligible to participate in the plan (in Age) \| Age	21
Employees hours of service scheduled to complete to be eligible to participate in the Plan (in Hours) \| Hours	1,000
Maximum [Member]
Description of Plan [Abstract]
Participant’s compensation percentage eligible for matching	3.50%
Maximum [Member] \| Balance Distributed Regardless if Participants Requested [Member]
Description of Plan [Abstract]
Vested account balance	$ 7,000

STATEMENTS OF NET ASSETS AVAILABLE FOR PLAN BENEFITS - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member] - USD ($)	Dec. 31, 2025	Dec. 31, 2024
Investments, at fair value:
Investments, at fair value	$ 411,209,167	$ 360,983,991
Receivables:
Notes receivable from participants	3,492,310	3,450,393
Due from broker	86,234	62,387
Contributions receivable from employer	675,053	1,987,973
Total receivables	4,253,597	5,500,753
Total assets	415,462,764	366,484,744
Liabilities
Due to broker	92,118	41,865
Total liabilities	92,118	41,865
Net assets available for plan benefits	415,370,646	366,442,879
Mutual Funds [Member]
Investments, at fair value:
Investments, at fair value	363,169,532	307,131,380
Common Stock of NBT Bancorp Inc. [Member]
Investments, at fair value:
Investments, at fair value	30,074,182	35,226,305
Stable Value Fund [Member]
Investments, at fair value:
Investments, at fair value	17,933,465	18,539,578
Cash and Money Market Funds [Member]
Investments, at fair value:
Investments, at fair value	$ 31,988	$ 86,728

Description of Plan	12 Months Ended
Description of Plan	Dec. 31, 2025
NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]
Description of Plan [Abstract]
Description of Plan	1. Description of Plan The following description of the NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan (“the Plan”) provides only general information. Participants should refer to the Plan Agreement or Summary Plan Description for a more complete description of the Plan’s provisions. General The Plan is a defined contribution plan as defined under Section 401(a) of the Internal Revenue Code (“IRC”), sponsored by NBT Bancorp Inc. (“the Sponsor”, “the Plan Administrator” or “the Company”). The Sponsor is responsible for administration of the Plan. NBT Bank, N.A, is a wholly-owned subsidiary of NBT Bancorp Inc. NBT Bank, N.A. is the trustee of the Plan (“the Trustee”) and Charles Schwab Bank is the Custodian of the Plan. EPIC Retirement Plan Services, a wholly‑owned subsidiary of NBT Financial Services, Inc., a wholly‑owned subsidiary of the Sponsor, is the Plan’s record keeper. The assets of the Plan are held, administered and managed in accordance with the terms and conditions of the Trust Agreement, which is considered to be an integral part of the Plan. The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”). The Plan was amended to include employees of Karl W. Reynard Agency effective July 1, 2024 and employees of Paco, Inc. effective November 1, 2024. The Plan was amended to include employees of Evans Bancorp, Inc. effective May 2, 2025. Effective January 1, 2025, the Plan was amended to update provisions related to employees acquired in IRC §410(b)(6)(C) transactions (including eligibility as of the transaction date and crediting of predecessor employer service for eligibility and vesting purposes). The Plan was also amended to permit in‑plan Roth conversions effective January 1, 2026. Employees acquired in certain IRC §410(b)(6)(C) transactions may become eligible as of the transaction date, as provided under the Plan. Eligibility All employees who are age 21 or over and scheduled to complete 1,000 hours of service or have completed 1,000 hours of service are eligible to participate in the Plan. Eligible employees are required to complete 30 days of employment prior to entry into the Plan. A deemed election becomes effective as of the first day of the month the eligible employee becomes a participant and is eligible to make a deferral election to the Plan. Contributions and Deferrals Participants may make pre‑tax and post-tax contributions in whole percentages or may elect a flat dollar contribution up to Internal Revenue Service (“IRS”) limitations for any plan year. The post-tax contributions are deferrals to Roth accounts. The Plan includes provisions for automatic elective contributions under which any employee that meets certain eligibility requirements will be automatically enrolled in the Plan and will automatically have 6% withheld from his or her compensation and contributed to the Plan. The employee will have to elect to opt out of the qualified automatic contribution election. The Company’s matching contribution is 100% of each participant’s contribution up to 1% of compensation plus 50% of the next 5% of compensation for a total matching contribution of up to 3.5% of compensation. The Company match amounted to $5,916,604 and $5,100,193 in 2025 and 2024, respectively. A discretionary amount, determined by the Sponsor’s Board of Directors, may be contributed to the Plan each year. To share in this discretionary contribution, participants must be actively employed on the last day of the year, have completed 1,000 hours of service and have contributed a minimum percentage of compensation during the year as determined annually by the Company. The amount is allocated to participants on a pro-rata basis, based on compensation. No discretionary contributions were made for 2025. During 2024, discretionary contributions of $1,422,473 were approved by the Sponsor’s board of directors and were paid during 2025. This discretionary contribution is included in contributions receivable from employer in the Statement of Net Assets Available for Plan Benefits as of December 31, 2024. In addition, as defined in the Plan document, employees participating in the Plan under the Worker Retirement Accumulation Plan (“WRAP”) design can receive an additional discretionary Company contribution equal to the interest credit on service credits earned under the WRAP design. The Company contribution for employees participating in the WRAP amounted to $675,053 in 2025, which was paid during 2026, and $565,500 in 2024, which was paid during 2025. These discretionary contributions are included in contributions receivable from employer in the Statement of Net Assets Available for Plan Benefits as of December 31, 2025 and 2024. Participant Accounts Participants elect to have their contributions invested among the various funds available to the Plan, including NBT Bancorp Inc. common stock. Each participant’s account is credited with the Sponsor’s and participant’s contributions, plan earnings and income, expenses, gains and losses attributable thereto. Vesting Participants’ contributions and net investment income or loss thereon are 100% vested. The participants’ vesting in Safe Harbor employer matching contributions are 100% vested upon completion of two years of service. Employer discretionary contributions are vested after five years of service. Participants are considered 100% vested upon termination due to death, retirement or permanent disability. Rollovers and Transfers from Other Qualifying Plans Participants may make rollover contributions to the Plan through a distribution from a former employer’s qualified retirement plan in accordance with the IRC. Notes Receivable from Participants Participants may borrow from their account in amounts ranging from $1,000 to the lesser of $50,000 or 50% of the vested 401(k) account balance excluding Company contributions invested in NBT Bancorp Inc. common stock and employer contributions made subsequent to January 1, 1997. Loans, other than loans for the purchase of a primary residence, must be repaid over a period no longer than five years. Loans for the purchase of a primary residence must be repaid over a period no longer than 15 years. Interest is charged at the prime rate plus 1% as of the loan origination date. Participant loans are treated as a transfer from the participant directed accounts into the loan fund. Principal and interest payments on the loans are allocated to the loan fund and transferred into the participant directed accounts based on the participants’ current investment allocation elections. Payment of Benefits Upon normal or early retirement, disability, death or termination of employment, the value of a participant’s account is paid as requested by the participant. If the value of the vested balance does not exceed $7,000, then the balance will be distributed regardless if the participant requests to receive it. Early retirement is allowed upon reaching age 55 and completion of at least 5 years of service. Withdrawals Subject to certain limitations prescribed by the Plan and the IRC, terminated participants may elect retirement or other termination withdrawals in either lump sum or partial payments. Forfeitures Forfeitures are applied to reduce the amount of future employer contributions otherwise required to be paid. In 2025 and 2024, forfeitures from non-vested accounts totaled $149,133 and $145,368, respectively, and forfeitures used to reduce employer contributions were $127,451 and $141,313, respectively. Forfeiture account balances totaled $66,394 and $44,712 at December 31, 2025 and 2024, respectively. Plan Termination Although it has not expressed any intention to do so, the Sponsor has the right to discontinue contributions or terminate the Plan at any time subject to Plan provisions and subject to the provisions of ERISA. In the event of termination of the Plan, each participant’s account would become fully vested and the net assets of the Plan would be allocated as prescribed by ERISA and its related regulations. Administrative Expenses Expenses of operating and administering the Plan are generally paid by the Sponsor. The payment of these expenses is not mandated by the Plan and is done so at the discretion of the Sponsor. Loan fees are paid by the borrower. Voting Rights With respect to participant account balances that are invested in shares of NBT Bancorp Inc. common stock, each participant is entitled to exercise voting rights attributable to the shares allocated to his or her account and is notified by the Trustee prior to the time that such rights are to be exercised.

Party-in-Interest Transactions (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member] - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Party-in-Interest Transactions [Abstract]
Total investments at fair value	$ 411,209,167	$ 360,983,991
Notes receivable from participants	3,492,310	3,450,393
Total dividend income	20,873,491	12,838,626
Company Common Stock [Member]
Party-in-Interest Transactions [Abstract]
Purchases of Company Stock	3,824,520	4,560,353
Sales of Company stock	$ 3,236,945	$ 6,099,149
Number of shares of Company stock held in participants accounts (in Shares)	724,330	737,569
Total investments at fair value	$ 30,074,182	$ 35,226,305
Total dividend income	$ 1,043,188	$ 1,022,466

- Definition

Fair value of investment, excluding plan interest in master trust, held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Includes, but is not limited to, derivative, investment measured at net asset value per share as practical expedient, and insurance contract not meeting fully-benefit responsive investment criteria.

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+ Details

Fair Value Measurements - Financial Instruments Recorded at Fair Value on a Recurring Basis (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member] - USD ($)	Dec. 31, 2025	Dec. 31, 2024
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	$ 411,209,167	$ 360,983,991
Mutual Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	363,169,532	307,131,380
Common Stock of NBT Bancorp Inc. [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	30,074,182	35,226,305
Stable Value Fund [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	17,933,465	18,539,578
Cash and Money Market Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	31,988	86,728
Level 1 [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	411,209,167	360,983,991
Level 1 [Member] \| Mutual Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	363,169,532	307,131,380
Level 1 [Member] \| Common Stock of NBT Bancorp Inc. [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	30,074,182	35,226,305
Level 1 [Member] \| Stable Value Fund [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	17,933,465	18,539,578
Level 1 [Member] \| Cash and Money Market Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	31,988	86,728
Level 2 [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 2 [Member] \| Mutual Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 2 [Member] \| Common Stock of NBT Bancorp Inc. [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 2 [Member] \| Stable Value Fund [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 2 [Member] \| Cash and Money Market Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 3 [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 3 [Member] \| Mutual Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 3 [Member] \| Common Stock of NBT Bancorp Inc. [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 3 [Member] \| Stable Value Fund [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	0	0
Level 3 [Member] \| Cash and Money Market Funds [Member]
Fair Value Measurements [Abstract]
Financial instruments recorded at fair value	$ 0	$ 0

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2025

NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]

Fair Value Measurements [Abstract]

Financial Instruments Recorded at Fair Value on a Recurring Basis

The following table presents the financial instruments recorded at fair value on a recurring basis by the Plan as of December 31, 2025 and 2024:


Description	December 31, 2025		Level 1			Level 2			Level 3
Mutual funds	$	363,169,532		$	363,169,532		$	-		$	-
Common stock of NBT Bancorp Inc.		30,074,182			30,074,182			-			-
Stable value fund		17,933,465			17,933,465			-			-
Cash and money market funds		31,988			31,988			-			-
Total	$	411,209,167		$	411,209,167		$	-		$	-


Description	December 31, 2024		Level 1		Level 2		Level 3
Mutual funds	$	307,131,380	$	307,131,380	$	-	$	-
Common stock of NBT Bancorp Inc.		35,226,305		35,226,305		-		-
Stable value fund		18,539,578		18,539,578		-		-
Cash and money market funds		86,728		86,728		-		-
Total	$	360,983,991	$	360,983,991	$	-	$	-

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Tabular disclosure of fair value of investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), including, but not limited to, fair value measured at net asset value per share as practical expedient (NAV).

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+ Details

Fair Value Measurements

12 Months Ended

Dec. 31, 2025

NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]

Fair Value Measurements [Abstract]

Fair Value Measurements

The following table presents the financial instruments recorded at fair value on a recurring basis by the Plan as of December 31, 2025 and 2024:


Description	December 31, 2025		Level 1			Level 2			Level 3
Mutual funds	$	363,169,532		$	363,169,532		$	-		$	-
Common stock of NBT Bancorp Inc.		30,074,182			30,074,182			-			-
Stable value fund		17,933,465			17,933,465			-			-
Cash and money market funds		31,988			31,988			-			-
Total	$	411,209,167		$	411,209,167		$	-		$	-


Description	December 31, 2024		Level 1		Level 2		Level 3
Mutual funds	$	307,131,380	$	307,131,380	$	-	$	-
Common stock of NBT Bancorp Inc.		35,226,305		35,226,305		-		-
Stable value fund		18,539,578		18,539,578		-		-
Cash and money market funds		86,728		86,728		-		-
Total	$	360,983,991	$	360,983,991	$	-	$	-

The stable value fund consists of the Federated Capital Preservation Fund (“the Fund”), which primarily holds guaranteed investment contracts (“GICs”) and synthetic guaranteed investment contracts (“synthetic GICs”). GICs represent deposits which guarantee a stated interest rate for the term of the contracts. The crediting rate of security-backed contracts will track current market yields on a trailing basis. The rate reset allows the contract value to converge with the fair value of the underlying portfolio over time, assuming the portfolio continues to earn the current yield for a period of time equal to the current portfolio duration. The fair value of GICs is determined based on the present value of the contract's expected cash flows, discounted by current market interest rates for like duration and like quality investments. Synthetic GICs are portfolios of securities (debt securities or open end registered investment companies) owned by the Fund with wrap contracts that guarantee a fixed or variable rate for the term of the contracts.

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

- Definition

The entire disclosure for fair value of investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), including, but not limited to, fair value measured at net asset value per share as practical expedient (NAV).

+ References

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+ Details

STATEMENTS OF CHANGES IN NET ASSETS AVAILABLE FOR PLAN BENEFITS - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member] - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Net investment income:
Net appreciation in fair value of investments	$ 23,419,439	$ 33,173,589
Dividends	20,873,491	12,838,626
Interest	568,966	528,817
Total net investment income	44,861,896	46,541,032
Interest income on notes receivable from participants	282,451	260,292
Contributions:
Participants	15,806,249	13,792,131
Employer	6,591,657	7,088,166
Rollovers	15,202,083	4,957,880
Total contributions	37,599,989	25,838,177
Total additions	82,744,336	72,639,501
Deductions
Distributions to participants	(33,787,979)	(26,959,027)
Administrative expenses	(28,590)	(25,806)
Total deductions	(33,816,569)	(26,984,833)
Net increase in net assets available for plan benefits	48,927,767	45,654,668
Net assets available for plan benefits
Beginning of year	366,442,879	320,788,211
End of year	$ 415,370,646	$ 366,442,879

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2025
NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]
Summary of Significant Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying financial statements have been prepared on the accrual basis of accounting in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Amounts in prior year’s financial statements are reclassified, when necessary, to conform with current year’s presentation. Investment Valuation and Income Recognition The Plan’s investments are stated at fair value. The fair value of mutual funds and the common stock of the Sponsor are based on published market quotations. The fair value of the stable value fund is based on the calculated daily net asset values of the fund. Fair value is the price the Plan would receive to sell an asset or pay to transfer a liability in an orderly transaction with a market participant at the measurement date. In the absence of active markets for the identical assets and liabilities, such measurements involve developing assumptions based on market observable data and, in the absence of such data, internal information that is consistent with what market participants would use in a hypothetical transaction that occurs at the measurement date. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect our market assumptions. Preference is given to observable inputs. These two types of inputs create the following fair value hierarchy and a financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The three levels are defined as follows: Level 1 - Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities; Level 2 - Quoted prices for similar assets or liabilities in active markets, quoted prices in markets that are not active, or inputs that are observable, either directly or indirectly, for substantially the full term of the asset or liability; and Level 3 - Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (e.g., supported by little or no market activity). Investment transactions are recorded on a trade date basis. If a trade is open at the end of the year, a receivable for securities sold but not yet settled or a payable for securities purchased but not yet settled is reflected in the Statement of Net Assets Available for Benefits. Dividends are recorded on the ex-dividend date. Interest income is earned from settlement date and recognized on the accrual basis. The net appreciation in the fair value of investments consists of the realized gains or losses on the sales of investments and the net unrealized appreciation of investments. The methods described above may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Furthermore, although the Plan believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date. Benefits Paid to Participants Benefit payments to participants are recorded when paid. Notes Receivable from Participants Notes receivable from participants are carried at the unpaid principal balance plus interest. Participant Withdrawals Participant withdrawals are recorded when paid. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of net assets available for plan benefits, disclosure of contingent assets and liabilities, and the reported amount of increases and decreases in net assets available for plan benefits. Actual results could differ from those estimates. Risks and Uncertainties The Plan invests in various types of investment securities. Investment securities are exposed to various risks, such as interest rate, market and credit risks. Due to the level of risk associated with certain investment securities, it is at least reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participant account balances and the amounts reported in the Statements of Net Assets Available for Plan Benefits.

SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]	12 Months Ended
	Dec. 31, 2025
SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract]
Plan name	401(k) AND EMPLOYEE STOCK OWNERSHIP PLAN
EIN#	16-1268674
Plan #	002

Document And Entity Information	12 Months Ended
Document And Entity Information	Dec. 31, 2025
Document Information Line Items
Entity Central Index Key	0000790359
Document Type	11-K
Document Period End Date	Dec. 31, 2025
Document Fiscal Year Focus	2025
Entity Registrant Name	NBT Bancorp Inc.
Amendment Flag	false
NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]
Document Information Line Items
Document Type	11-K
Document Period End Date	Dec. 31, 2025
Document Fiscal Year Focus	2025
Amendment Flag	false

Income Tax Status	12 Months Ended
Income Tax Status	Dec. 31, 2025
NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member]
Income Tax Status [Abstract]
Income Tax Status	4. Income Tax Status The Plan operates under an IRS pre-approved plan document, as administered by EPIC Retirement Plan Services. The latest IRS favorable opinion letter on the pre-approved plan document is dated June 30, 2020. Although the opinion letter is dated prior to the Plan’s amendment, the Plan Administrator and the Plan’s tax counsel believe that the Plan is designed and is currently being operated, in compliance with the applicable requirements of the IRC and, therefore, believe that the Plan is qualified and the related trust is tax-exempt.