Legal Entity [Axis] STATEMENTS OF NET ASSETS AVAILABLE FOR PLAN BENEFITS [Abstract] Investment Type [Axis] Total receivables Total assets Total liabilities Net assets available for plan benefits Beginning of year End of year STATEMENTS OF CHANGES IN NET ASSETS AVAILABLE FOR PLAN BENEFITS [Abstract] Total net investment income Total contributions Total additions Total deductions Net increase in net assets available for plan benefits Description of Plan [Abstract] Summary of Significant Accounting Policies [Abstract] Fair Value Measurements [Abstract] Fair Value Hierarchy and NAV [Axis] Income Tax Status [Abstract] Party-in-Interest Transactions [Abstract] SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract] Statistical Measurement [Axis] Document Type Document Period End Date Document Fiscal Year Focus Entity Registrant Name Entity Central Index Key Amendment Flag Assets Investments, at fair value: Investments, at fair value Financial instruments recorded at fair value Total investments at fair value Receivables: Notes receivable from participants Due from broker Contributions receivable from employer Liabilities Due to broker Additions Net investment income: Net appreciation in fair value of investments Dividends Total dividend income Interest Interest income on notes receivable from participants Contributions: Participants Employer Company contributions Rollovers Deductions Net assets available for plan benefits Distributions to participants Administrative expenses NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member] Mutual Funds [Member] Common Stock of NBT Bancorp Inc. [Member] Company Common Stock [Member] Stable Value Fund [Member] Cash and Money Market Funds [Member] Description of Plan Summary of Significant Accounting Policies Fair Value Measurements Financial Instruments Recorded at Fair Value on a Recurring Basis Income Tax Status Party-in-Interest Transactions SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) Identity of issuer, borrower, lessor, or similar party Participant directed status Current value Description of investment Party-in-interest Number of shares (in Shares) Number of shares of Company stock held in participants accounts (in Shares) Participant loans receivable Participant loans receivable, Interest rate Asset held for investment Level 1 [Member] Level 2 [Member] Level 3 [Member] Investment, Identifier [Axis] Participant loans receivable [Member] Minimum [Member] Maximum [Member] EBP, Contribution, Participating Employer [Axis] Plan agreement description Employees age limit to be eligible to participate in the plan (in Age) Employees hours of service scheduled to complete to be eligible to participate in the Plan (in Hours) Employees hours of service completed to be eligible to participate in the Plan (in Hours) Employment period required for eligibility to participate in plan Percentage automatically withheld from employee compensation and contributed to the Plan Company's matching contribution percentage Participant’s compensation percentage eligible for matching Participants' contributions vesting percentage Employee vesting period Employee benefit plan participants contributions vesting period Minimum amount participants may borrow from their vested accounts Maximum amount participants may borrow from their vested accounts Maximum percentage participants may borrow from their vested accounts Notes receivable from participants, repayment term Notes receivable from participants, Spread on Prime Rate Vested account balance Early retirement age Early retirement minimum service period Forfeiture from non-vested account balances Forfeitures used to reduce employer contributions Forfeiture from account balances IRS favorable opinion letter IRS favorable opinion letter, date Tax-exempt qualified status Purchases of Company Stock Sales of Company stock Plan name EIN# Plan # Basis of Presentation Investment Valuation and Income Recognition Benefits Paid to Participants Notes Receivable from Participants Participant Withdrawals Use of Estimates Risks and Uncertainties Participant’s Contribution Eligible for 100% Matching [Member] Participant’s Contribution From 1% to 5% Eligible for 50% Matching [Member] Company Matching [Member] Discretionary Contributions [Member] Worker Retirement Accumulation Plan [Member] Participants' Contributions and Net Investment Income of Loss [Member] Safe Harbor Employer Matching Contributions [Member] Termination Due to Death, Retirement or Permanent Disability [Member] Loans, Other Than Loans for Purchase of Primary Residence [Member] Loans for Purchase of Primary Residence [Member] Balance Distributed Regardless if Participants Requested [Member] Investments [Domain] Entity [Domain] EBP, Statement of Net Asset Available for Benefit [Table] STATEMENTS OF NET ASSETS AVAILABLE FOR PLAN BENEFITS [Abstract] EBP, Statement of Change in Net Asset Available for Benefit [Table] STATEMENTS OF CHANGES IN NET ASSETS AVAILABLE FOR PLAN BENEFITS [Abstract] EBP, Description of Plan [Table] Description of Plan [Abstract] EBP, Accounting Policy [Table] Summary of Significant Accounting Policies [Abstract] EBP, Investment, Fair Value and NAV [Table] Fair Value Measurements [Abstract] EBP, Tax Status [Table] Income Tax Status [Abstract] EBP, Related Party and Party-in-Interest Transactions [Table] Party-in-Interest Transactions [Abstract] EBP, Schedule of Asset Held for Investment [Table] SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) [Abstract] EBP, Accounting Policy [Line Items] Fair Value Hierarchy and NAV [Domain] EBP, Investment, Fair Value and NAV [Table] Statistical Measurement [Domain] EBP, Contribution, Participating Employer [Domain] Represents participant early retirement age under employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Represent participant early retirement minimum service period under employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). The percentage of employee contributions that are immediately vested in an employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Amount of plan asset, in forfeited account, no longer allocated to participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). The period over which a participant loan (note receivable from participant account) is required to be repaid under the terms of the plan. The amount of spread, margin, or percentage points above or below a referenced variable interest rate used to determine the interest rate charged on participant notes receivable. Participant age limit to participate in the employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). The period of employment an employee is required to complete before becoming eligible to participate in the plan. The number of hours of service a participant must complete in order to become eligible to participate in the plan. Participant eligible to participate upon completing the schedules service in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Represents the minimum period of service required for a plan participant to become fully vested in the employer contributions under employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). The participant's vested balance without requiring the participant's consent or request. If the vested balance does not exceed this threshold amount, the plan must distribute the balance regardless of participant under employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Represents the period of time over which an employee must remain employed before becoming fully vested in the employer's contributions or benefits under employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Disclosure of accounting policy for participant withdrawals of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). A stable value fund is a low-risk investment option designed primarily to preserve principal while generating steady interest. Exclusively available in tax-advantaged retirement plans (like 401(k)s and 403(b)s), they invest in high-quality bonds and use insurance contracts to guarantee a stable, constant share price. NBT Bancorp Inc. [Member] Mutual Fund [Member] Federated Hermes Capital Preservation Fund [Member] American Fund EUPAC Fund [Member] Participant-Directed Investments [Member] Cash [Member] T-Rowe Price Small-Capital Value Fund [Member] T-Rowe Price Retirement 2060 Fund [Member] T-Rowe Price Retirement 2030 Fund [Member] Interest rates [Member] Qualified Plan [Member] Stable value fund [Member] American Funds New Perspective Fund [Member] T-Rowe Price Retirement Balance Fund [Member] Dodge & Cox Income Fund [Member] Vanguard Mid-Cap Index Fund Institutional [Member] DFA Emerging Markets Portfolio Institutional [Member] Columbia Dividend Income Fund S [Member] Money Market Funds [Member] T-Rowe Price Mid Cap-Growth Fund [Member] Fidelity 500 Index Fund [Member] Common Stock [Member] Vanguard Short Term Bond Index Fund Admiral [Member] Vanguard Balanced Index Fund Institutional [Member] T-Rowe Price Retirement 2020 Fund [Member] DFA US Sustainability Core 1 Portfolio [Member] T-Rowe Price Retirement 2050 Fund [Member] T-Rowe Price Retirement 2010 Fund [Member] T-Rowe Price Dividend Growth Fund [Member] Vanguard Intermediate US Treasury [Member] T-Rowe Price Retirement 2040 Fund [Member] Schwab Value Advantage Money Fund [Member] Fidelity Inflation Protected Bond Index Fund [Member] T-Rowe Price Growth Stock Fund [Member] Participant Loans Receivable Member EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Participants Contribution From1 To5 Eligible For50 Matching Member EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Participant Contribution, Automatic, Deferral Rate EBP, Employer Contribution, Participant Compensation Matched, Percentage Fair Value, Inputs, Level 3 [Member] EBP, Tax Qualification Status [Extensible Enumeration] EBP, Employer Contribution, Matching Percentage EBP, Plan Number Worker Retirement Accumulation Plan Member EBP, Risk and Uncertainty [Policy Text Block] EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] EBP, Note Receivable from Participant [Policy Text Block] EBP, Investment, Fair Value and NAV [Table Text Block] EBP, Related Party and Party-in-Interest Transactions [Text Block] EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount Employee Benefit Plan Note Receivable From Participant Account Repayment Term Employee Benefit Plan Participant Age Limit To Participate In Plan EBP, Description, Available [true false] Employee Benefit Plan Participant Service Completed To Be Eligible To Participate In Plan EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment EBP, Related Party and Party-in-Interest Transactions [Line Items] EBP002 Member Fair Value, Inputs, Level 1 [Member] EBP, Change in Net Asset Available for Benefit, Increase EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense EBP, Summary of Accounting Policy [Text Block] EBP, Liability for Investment Purchase EBP, Tax Status [Text Block] EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment [Abstract] EBP, Note Receivable from Participant EBP, Receivable EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Discretionary Contributions Member EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Investment EBP, Change in Net Asset Available for Benefit, Increase from Contribution Safe Harbor Employer Matching Contributions Member Employee Benefit Plan Participants Vested Account Value Employee Benefit Plan Early Retirement Age Participants Contribution Eligible For100 Matching Member EBP, Liability EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount EBP, Investment, Current Value, Type [Extensible Enumeration] EBP, Asset [Abstract] EBP, Use of Estimate [Policy Text Block] EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment Participants Contributions And Net Investment Income Of Loss Member EBP, Asset EBP, Investment Acquired, Excluding Plan Interest in Master Trust, Cost EBP, Investment, Fair Value [Abstract] EBP, Change in Net Asset Available for Benefit, Increase [Abstract] EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution EBP, Description of Plan [Text Block] Loans For Purchase Of Primary Residence Member Document Fiscal Year Focus Employee Benefit Plan Note Receivable From Participant Account Spread On Variable Rate EBP, Tax Determination Letter, Date EBP, Asset Held for Investment, Participant Loan, Interest Rate Loans Other Than Loans For Purchase Of Primary Residence Member EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage EBP, Forfeited Nonvested Account Minimum [Member] EBP, Investment, Issuer Name [Extensible Enumeration] EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] EBP, Investment, Number of Shares EBP, Description of Plan [Line Items] EBP, Asset Held for Investment, Party-in-Interest [true false] Balance Distributed Regardless If Participants Requested Member EBP, Net Asset Available for Benefit [Abstract] EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash Employee Benefit Plan Participant Service Scheduled To Complete To Be Eligible To Participate In Plan Fair Value, Inputs, Level 2 [Member] Termination Due To Death Retirement Or Permanent Disability Member EBP, Investment Sold, Excluding Plan Interest in Master Trust, Selling Price Employee Benefit Plan Employee Contribution Vesting Percentage EBP, Change in Net Asset Available for Benefit, Decrease EBP, Basis of Accounting [Policy Text Block] EBP, Investment, Fair Value and NAV [Line Items] EBP, Investment Sale, Receivable EBP, Liability [Abstract] EBP, Asset Held for Investment EBP, Receivable [Abstract] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Net Asset Available for Benefit EBP, Investment [Policy Text Block] Maximum [Member] EBP, Schedule of Asset Held for Investment [Line Items] Document Type EBP, Employer Contribution, Receivable Entity Tax Identification Number Amendment Flag EBP, Tax Status [Line Items] Document Period End Date Cash and Cash Equivalents [Member] Employee Benefit Plan Participant Employment Period Required For Eligibility To Participate In Plan EBP, Investment, Fair Value and NAV [Text Block] EBP, Schedule of Asset Held for Investment [Table Text Block] Employee Benefit Plan Vesting Period Entity Central Index Key EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value Employee Benefit Plan Participants Contributions Vesting Period Investment, Identifier [Axis] EBP, Forfeited Nonvested Account, Decrease from Employer Contribution Participant Withdrawals Policy Text Block EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover EBP, Plan Name Employee Benefit Plan Early Retirement Minimum Service Period EBP, Payment to Participant [Policy Text Block] Employee Benefit Plan Forfeited From Account Entity Registrant Name EBP, Change in Net Asset Available for Benefit, Increase (Decrease) EBP, Tax Determination Letter, Obtained [true false] Company Matching Member