Loans outstanding EBP, Loan Outstanding EBP, Loan Outstanding Entity tax identification number Entity Tax Identification Number Employee rollover EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Maximum Maximum [Member] Forfeited nonvested account, decrease from administrative expense EBP, Forfeited Nonvested Account, Decrease from Administrative Expense Legal Entity [Axis] Legal Entity [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] EBP, Description of Plan [Table] EBP, Description of Plan [Table] Mutual funds Mutual Fund [Member] 4 years EBP, Service Period, 4 Years [Member] EBP, Service Period, 4 Years EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Table] Participant contribution, pretax, maximum annual compensation to total compensation, percentage EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Employee, years of service EBP, Employee, Years Of Service EBP, Employee, Years Of Service Total deductions EBP, Change in Net Asset Available for Benefit, Decrease Self-directed brokerage account EBP, Self-Directed Brokerage Account [Member] Note receivable from participant, account, maximum borrowing, amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Fair Value Measurements EBP, Investment, Fair Value and NAV [Text Block] Total investments at fair value EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value EBP, Service Period [Axis] EBP, Service Period [Axis] EBP, Service Period Level 1 Fair Value, Inputs, Level 1 [Member] EBP, Investment, Fair Value and NAV [Abstract] Assets EBP, Asset [Abstract] Statistical Measurement [Axis] Statistical Measurement [Axis] Additions EBP, Change in Net Asset Available for Benefit, Increase [Abstract] EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Statement of Change in Net Asset Available for Benefit [Table] 5 years or more EBP, Service Period, 5 Years Or More [Member] EBP, Service Period, 5 Years Or More Payment of Benefits EBP, Payment to Participant [Policy Text Block] Transfers into Plan EBP, Change in Net Asset Available for Benefit, Transfer to (from) Plan Amendment Flag Amendment Flag Basis of Accounting EBP, Basis of Accounting [Policy Text Block] EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Table] Investment, number of shares (in shares) Description of Investment Including Maturity Date, Rate of Interest, Collateral, Par, or Maturity Value (in shares) EBP, Investment, Number of Shares Actual interest credited EBP, Investment, Interest Rate EBP, Description of Plan and Accounting Change [Abstract] Schedule of Investment, Fair Value and NAV EBP, Investment, Fair Value and NAV [Table Text Block] EBP 002 EBP 002 [Member] EBP 002 Investments [Domain] Investments [Domain] Use of Estimates EBP, Use of Estimate [Policy Text Block] Participant contribution, automatic, annual increase deferral rate EBP, Participant Contribution, Automatic, Annual Increase Deferral Rate EBP, Participant Contribution, Automatic, Annual Increase Deferral Rate Pooled separate account EBP, Pooled Separate Account [Member] Schedule H, Part IV, Line 4i - Schedule of Assets (Held at End of Year) EBP, Schedule of Asset Held for Investment [Text Block] Cover [Abstract] Employer EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash Minimum Minimum [Member] Investment Type [Axis] Investment Type [Axis] Total contributions EBP, Change in Net Asset Available for Benefit, Increase from Contribution Employee EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Net appreciation in fair value of investments EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment Interest and dividend income EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment EBP, Description of Plan [Line Items] EBP, Description of Plan [Line Items] Plan number EBP, Plan Number EBP, Investment, Fair Value and NAV [Line Items] EBP, Investment, Fair Value and NAV [Line Items] Description of Plan EBP, Description of Plan [Text Block] Guaranteed Annuity Contract with Principal Life Insurance Company EBP, Guaranteed Annuity Contract [Text Block] EBP, Guaranteed Annuity Contract EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Schedule of Asset Held for Investment [Line Items] EBP, Schedule of Asset Held for Investment [Line Items] Common Stock of Northfield Bancorp, Inc. EBP, Employer, Common Stock [Member] EBP, Schedule of Asset Held for Investment [Abstract] EBP, Accounting Policy [Table] EBP, Accounting Policy [Table] Forfeited nonvested, percent termination EBP, Forfeited Nonvested Account, Percent Termination EBP, Forfeited Nonvested Account, Percent Termination Total additions to net assets EBP, Change in Net Asset Available for Benefit, Increase Investment income EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment [Abstract] Investment, Identifier [Axis] Investment, Identifier [Axis] Administrative expenses EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Risks and Uncertainties EBP, Risk and Uncertainty [Policy Text Block] Document Information [Line Items] Document Information [Line Items] Employer contribution, participant compensation matched, percentage EBP, Employer Contribution, Participant Compensation Matched, Percentage EBP, Statement of Change in Net Asset Available for Benefit [Abstract] Level 2 Fair Value, Inputs, Level 2 [Member] Employer contribution vesting percentage Percentage Vested EBP, Employer Contribution Vesting Percentage EBP, Employer Contribution Vesting Percentage EBP, Tax Status [Table] EBP, Tax Status [Table] Net increase in net assets EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Party-in-Interest Transactions EBP, Related Party and Party-in-Interest Transactions [Text Block] EBP, Related Party and Party-in-Interest Transactions [Abstract] EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] Participant contribution, automatic, deferral rate EBP, Participant Contribution, Automatic, Deferral Rate Investments at fair value: EBP, Investment, Fair Value [Abstract] Participant distributions EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Contract administrator fees EBP, Investment, Administrator Fee EBP, Investment, Administrator Fee Current Value EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] EBP, Statement of Net Asset Available for Benefit [Abstract] EBP, Accounting Policy [Line Items] EBP, Accounting Policy [Line Items] Principal Life Insurance Company Other Security Investments [Member] 2 years EBP, Service Period, 2 Years [Member] EBP, Service Period, 2 Years EBP, Tax Status [Abstract] Less than 1 year EBP, Service Period, Less Than 1 Year [Member] EBP, Service Period, Less Than 1 Year Notes receivable from participants, current value EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Investment Valuation and Income Recognition EBP, Investment [Policy Text Block] Fair Value, Inputs, Level 1, Level 2, and Level 3 Fair Value, Inputs, Level 1, Level 2, and Level 3 [Member] Document Type Document Type Participant contribution, pretax, minimum annual compensation to total compensation, percentage EBP, Participant Contribution, Pretax, Minimum Annual Compensation to Total Compensation, Percentage EBP, Participant Contribution, Pretax, Minimum Annual Compensation to Total Compensation, Percentage Entity Central Index Key Entity Central Index Key Notes receivable from participants, interest rate EBP, Asset Held for Investment, Participant Loan, Interest Rate Common collective trust funds Common Collective Trust [Member] EBP, Tax Status [Line Items] EBP, Tax Status [Line Items] EBP, Subsequent Event [Table] EBP, Subsequent Event [Table] Deductions EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] Notes receivable from participants EBP, Note Receivable from Participant Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Summary of Significant Accounting Policies EBP, Summary of Accounting Policy [Text Block] Entity Registrant Name Entity Registrant Name 1 year EBP, Service Period, 1 Year [Member] EBP, Service Period, 1 Year EBP, Subsequent Event [Abstract] Subsequent Events EBP, Subsequent Event [Text Block] 3 years EBP, Service Period, 3 Years [Member] EBP, Service Period, 3 Years Tax Status EBP, Tax Status [Text Block] Note receivable from participant, account, maximum borrowing, to total account, percentage EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage EBP, Service Period [Domain] EBP, Service Period [Domain] EBP, Service Period [Domain] EBP, Subsequent Event [Line Items] EBP, Subsequent Event [Line Items] Document Information [Table] Document Information [Table] Notes Receivable from Participants EBP, Note Receivable from Participant [Policy Text Block] Total investment income EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment EBP, Related Party and Party-in-Interest Transactions [Line Items] EBP, Related Party and Party-in-Interest Transactions [Line Items] Level 3 Fair Value, Inputs, Level 3 [Member] Net assets available for benefits Net assets available for benefits, beginning of the year Net assets available for benefits, end of the year EBP, Net Asset Available for Benefit Total investments EBP, Investment, Excluding Plan Interest in Master Trust, Fair and Contract Value Entity [Domain] Entity [Domain] Contributions EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] Investment at contract value - Guaranteed Annuity Contract EBP, Investment, Excluding Plan Interest in Master Trust, Contract Value Forfeited nonvested account EBP, Forfeited Nonvested Account Investments measured at Net Asset Value Fair Value Measured at Net Asset Value Per Share [Member] EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Line Items] Schedule of Vesting for Matching Contributions, Discretionary Contributions, and Related Earnings EBP, Schedule Of Vesting For Matching Contributions, Discretionary Contributions, And Related Earnings [Table Text Block] EBP, Schedule Of Vesting For Matching Contributions, Discretionary Contributions, And Related Earnings