Document and Entity Information [Abstract] Document Information [Table] Legal Entity [Axis] Entity [Domain] Represents the Retirement Plan 003. E B P 003 [Member] Needham Bank 401(k) Plan Document Information [Line Items] Document Type Document Period End Date Entity Registrant Name Entity Central Index Key Document Fiscal Year Focus Amendment Flag Statements of Net Assets Available for Benefits EBP, Statement of Net Asset Available for Benefit [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] Statements of Net Assets Available for Benefits EBP, Asset [Abstract] ASSETS EBP, Investment, Fair and Contract Value [Abstract] Investments: EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value Total investments at fair value Investments at fair value EBP, Investment, Excluding Plan Interest in Master Trust, Contract Value Investments at contract value EBP, Investment, Excluding Plan Interest in Master Trust, Fair and Contract Value Total investments EBP, Receivable [Abstract] Receivables: EBP, Note Receivable from Participant Notes receivable from participants EBP, Employer Contribution, Receivable Employer contribution EBP, Receivable Total receivables EBP, Net Asset Available for Benefit End of year Beginning of year Net assets available for benefits Statement of Changes in Net Assets Available for Benefits EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] Statement of Changes in Net Assets Available for Benefits EBP, Change in Net Asset Available for Benefit, Increase [Abstract] Additions: EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment [Abstract] Investment income: EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Unrealized Gain (Loss) on Investment Net appreciation in fair value of investments EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment Dividends and interest earned on investments EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment Total investment income EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Interest income on notes receivable from participants EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] Contributions: EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash and Noncash Employer EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Participants EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Rollovers EBP, Change in Net Asset Available for Benefit, Increase from Contribution Total contributions EBP, Change in Net Asset Available for Benefit, Increase Total additions EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] Deductions: EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Benefits paid to participants EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Administrative expenses Custodian fees EBP, Change in Net Asset Available for Benefit, Decrease Total deductions EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Net increase EBP, Change in Net Asset Available for Benefit, Increase (Decrease) [Abstract] Net assets available for benefits: DESCRIPTION OF PLAN EBP, Description of Plan [Table] EBP, Description of Plan [Line Items] PLAN TERMINATION EBP, Description of Plan [Text Block] DESCRIPTION OF PLAN SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES EBP, Accounting Policy [Table] EBP, Accounting Policy [Line Items] SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES EBP, Summary of Accounting Policy [Text Block] SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES FAIR VALUE MEASUREMENTS EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Line Items] FAIR VALUE MEASUREMENTS EBP, Investment, Fair Value and NAV [Text Block] FAIR VALUE MEASUREMENTS GROUP ANNUITY CONTRACT EBP, Fully Benefit-Responsive Investment Contract [Table] EBP, Fully Benefit-Responsive Investment Contract [Line Items] GROUP ANNUITY CONTRACT EBP, Fully Benefit-Responsive Investment Contract [Text Block] GROUP ANNUITY CONTRACT TAX STATUS EBP, Tax Status [Table] EBP, Tax Status [Line Items] TAX STATUS EBP, Tax Status [Text Block] TAX STATUS RISKS AND UNCERTAINTIES EBP, Risk and Uncertainty [Table] EBP, Risk and Uncertainty [Line Items] RISKS AND UNCERTAINTIES EBP, Risk and Uncertainty [Text Block] RISKS AND UNCERTAINTIES RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Line Items] RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS EBP, Related Party and Party-in-Interest Transactions [Text Block] RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS No definition available. PLAN TERMINATION EBP, Plan Termination [Text Block] PLAN TERMINATION SUBSEQUENT EVENTS EBP, Subsequent Event [Table] EBP, Subsequent Event [Line Items] SUBSEQUENT EVENTS EBP, Subsequent Event [Text Block] SUBSEQUENT EVENTS Schedule H, Line 4i - Schedule of Assets (Held at End of Year) EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Line Items] Schedule H, Line 4i - Schedule of Assets (Held at End of Year) EBP, Schedule of Asset Held for Investment [Text Block] Schedule H, Line 4i - Schedule of Assets (Held at End of Year) EBP, Basis of Accounting [Policy Text Block] Basis of Presentation EBP, Use of Estimate [Policy Text Block] Use of Estimates EBP, Investment [Policy Text Block] Investment Valuation and Income Recognition EBP, Contribution [Policy Text Block] Contributions EBP, Note Receivable from Participant [Policy Text Block] Notes Receivable from Participants EBP, Payment to Participant [Policy Text Block] Benefit Payments EBP, Expense [Policy Text Block] Administrative Expenses EBP, Investment, Fair Value and NAV [Table Text Block] Schedule of Plan's Assets Fair Value Hierarchy Represents the minimum age criteria for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Age For Plan Eligibility Minimum age for Plan eligibility Represents the minimum service period for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Service Period For Plan Eligibility Minimum service period for Plan eligibility Represents the threshold of minimum service hours to be considered a part-time, temporary, or seasonal employees for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Threshold of Minimum Service Hours of Part Time, Temporary, or Seasonal Employees For Plan Eligibility Threshold minimum service hours for plan eligibility Represents the minimum service hours of part-time, temporary, or seasonal employees who are regularly scheduled to work less than 1,000 hours of service for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Service Hours of Part Time, Temporary, or Seasonal Employees For Plan Eligibility Minimum service hours for plan eligibility Represents the minimum service period of part-time, temporary, or seasonal employees who are regularly scheduled to work less than 1,000 hours of service for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Service Period of Part Time, Temporary, or Seasonal Employees For Plan Eligibility Minimum service period of part time, temporary or seasonal employees for plan eligibility EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Employee contribution (in percentage) EBP, Participant Contribution, Automatic, Deferral Rate Deferral rate Percentage of automatic increase per year in pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Participant Contribution, Automatic Increase Per Year, Percent Increase in automatic deferral contributions, Percent Percentage of maximum automatic salary deferral contribution rate under automatic enrollment. Employee Benefit Plan, Participant Contribution, Maximum Automatic Enrollment Contribution, Percent Maximum automatic deferral contributions, percent EBP, Employer Contribution, Matching Percentage Employer contribution (in percentage) EBP, Employer Contribution, Participant Compensation Matched, Percentage Percentage of participant eligible compensation Percentage vested in eligible matching compensation contributed for participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) after vesting period has been completed. Employee Benefit Plan, Employer Matching Contribution, Vesting Percent Matching contribution, vesting percentage The period of service a participant must complete before becoming eligible to receive matching employer contributions under the (employee benefit plan (EBP)). Employee Benefit Plan, Employer Matching Contribution, Vesting Period Matching contribution, vesting period EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount Note receivable, minimum borrowing amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Note receivable, maximum borrowing amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage Note receivable, maximum borrowing percentage Term of participant loan of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Note Receivable From Participant, Term Loan term Number of notes outstanding a participant may borrow at any time in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Note Receivable from Participant, Account, Maximum Borrowing, Number of Loans Maximum number of loan outstanding allowed Represents the maximum vested balance for single lump sum payment in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Maximum Vested Balance For Single Lump Sum Payment Maximum vested balance for single lump sum payment EBP, Forfeited Nonvested Account Forfeited unvested accounts EBP, Forfeited Nonvested Account, Decrease from Employer Contribution Reduction in employer contribution from forfeited unvested accounts Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value, Inputs, Level 1 [Member] Level 1 Fair Value, Inputs, Level 2 [Member] Level 2 Investment Type [Axis] Investments [Domain] Mutual Fund [Member] Mutual funds Common Collective Trust [Member] Collective investment trusts EBP, Pooled Separate Account [Member] Pooled separate accounts EBP, Self-Directed Brokerage Account [Member] Self-directed brokerage accounts EBP, Employer, Common Stock [Member] Common stock EBP, Fair Value, Investment, NAV, Unfunded Commitment Unfunded commitments EBP, Traditional Guaranteed Investment Contract [Member] Traditional investment contract Statistical Measurement [Axis] Statistical Measurement [Domain] Minimum [Member] Minimum Maximum [Member] Maximum Annual crediting interest rate applied by the issuer of a fully benefit-responsive investment contract to determine earnings credited to contract value during the reporting period. Employee Benefit Plan, Fully Benefit Responsive Investment Contract, Guaranteed Interest Rate Crediting interest rate EBP, Tax Determination Letter, Obtained [true false] Determination letter obtained EBP, Tax Determination Letter, Date Determination letter date EBP, Tax Qualification Status [Extensible Enumeration] Tax qualification status EBP, Related Party and Party-in-Interest [Axis] EBP, Related Party and Party-in-Interest [Domain] EBP, Trustee [Member] Custodian Employer of participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Employer [Member] NB Bancorp, Inc EBP, Investment, Number of Shares Number of shares held Represents the percentage of vesting of participant accounts in the event of termination. Employee Benefit Plan, Vesting Percentage in Event of Termination Vesting percentage Investment, Identifier [Axis] EBP, Plan Name EBP, Plan Name Entity Tax Identification Number Entity tax identification number EBP, Plan Number Plan number EBP, Asset Held for Investment, Party-in-Interest [true false] EBP, Asset Held for Investment, Party-in-Interest [true false] EBP, Investment, Issuer Name [Extensible Enumeration] Investment issuer name EBP, Investment, Current Value, Type [Extensible Enumeration] EBP, Investment, Current Value, Type [Extensible Enumeration] EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value Current Value EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Notes receivable from participants EBP, Asset Held for Investment, Participant Loan, Interest Rate Interest rates of notes receivable from participants EBP, Asset Held for Investment Total Investments and Loans EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] EBP, Investment, Acquired, Number of Investment Investment assets acquired EBP, Investment, Sold, Number of Investments Investment assets disposed