Consolidated Financial Statements (Policies)	9 Months Ended
Sep. 30, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The consolidated financial statements are comprised of the accounts of Northfield Bancorp, Inc. and its wholly owned subsidiaries, Northfield Investments, Inc. and Northfield Bank (the “Bank”), and the Bank’s wholly-owned subsidiaries, NSB Services Corp. and NSB Realty Trust (collectively the “Company”). All significant intercompany accounts and transactions have been eliminated in consolidation. In the opinion of management, all adjustments (consisting solely of normal and recurring adjustments) necessary for the fair presentation of the consolidated balance sheets and the consolidated statements of comprehensive income for the unaudited periods presented have been included. The results of operations and other data presented for the three and nine months ended September 30, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022 or for any other period. Whenever necessary, certain prior year amounts are reclassified to conform to the current year presentation. In preparing the unaudited consolidated financial statements in conformity with U.S. generally accepted accounting principles (“U.S. GAAP”), management has made estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the consolidated balance sheets and for the periods indicated in the consolidated statements of comprehensive income. Material estimates that are particularly susceptible to change are: the allowance for credit losses, estimated cash flows of our purchased credit-deteriorated (“PCD”, or, previously, purchased credit-impaired “PCI”) loans and the valuation allowance against deferred tax assets. Estimates and assumptions are reviewed periodically and the effects of revisions are reflected in the consolidated financial statements in the period they are deemed necessary. While management uses its best judgment, actual amounts or results could differ significantly from those estimates. Certain information and note disclosures usually included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for the preparation of interim financial statements. The consolidated financial statements presented should be read in conjunction with the audited consolidated financial statements and notes to consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the SEC.
Fair Value Measurements	The following tables present the assets reported on the consolidated balance sheets at their estimated fair value as of September 30, 2022, and December 31, 2021, by level within the fair value hierarchy as required by the Fair Value Measurements and Disclosures Topic of the FASB ASC. Financial assets and liabilities are classified in their entirety based on the level of input that is significant to the fair value measurement. The fair value hierarchy is as follows: •Level 1 Inputs – Unadjusted quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. •Level 2 Inputs – Inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. These include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability (for example, interest rates, volatilities, prepayment speeds, loss severities, credit risks and default rates) or inputs that are derived principally from or corroborated by observable market data by correlations or other means. •Level 3 Inputs – Significant unobservable inputs that reflect the Company’s own assumptions that market participants would use in pricing the assets or liabilities.
Fair Value of Financial Instruments	The FASB ASC Topic for Financial Instruments requires disclosure of the fair value of financial assets and financial liabilities, including those financial assets and financial liabilities that are not measured and reported at fair value on a recurring or non-recurring basis. The methodologies for estimating the fair value of financial assets and financial liabilities that are measured at fair value on a recurring or non-recurring basis are discussed above. The following methods and assumptions were used to estimate the fair value of other financial assets and financial liabilities not already discussed above: (a) Cash and Cash Equivalents Cash and cash equivalents are short-term in nature with original maturities of three months or less; the carrying amount approximates fair value. Certificates of deposit having original terms of six-months or less; the carrying value generally approximates fair value. Certificates of deposit with an original maturity of six months or greater; the fair value is derived from discounted cash flows. (b) Debt Securities (Held-to-Maturity) The estimated fair values for substantially all of our securities are obtained from an independent nationally recognized pricing service. The independent pricing service utilizes market prices of same or similar securities whenever such prices are available. Prices involving distressed sellers are not utilized in determining fair value. Where necessary, the independent third-party pricing service estimates fair value using models employing techniques such as discounted cash flow analysis. The assumptions used in these models typically include assumptions for interest rates, credit losses, and prepayments, utilizing market observable data where available. (c) Investments in Equity Securities at Net Asset Value Per Share The Company uses net asset value as a practical expedient to record its investment in a private SBA Loan Fund since the shares in the fund are not publicly traded, do not have a readily determinable fair value and the net asset value per share is calculated in a manner consistent with the measurement principles of an investment company. (d) Federal Home Loan Bank of New York Stock The fair value for Federal Home Loan Bank of New York ("FHLBNY") stock is its carrying value, since this is the amount for which it could be redeemed and there is no active market for this stock. (e) Loans (Held-for-Investment) Fair values are estimated for portfolios of loans with similar financial characteristics. Loans are segregated by type such as originated and purchased, and further segregated by residential mortgage, construction, land, multifamily, commercial and consumer. Each loan category is further segmented into amortizing and non-amortizing and fixed and adjustable rate interest terms and by performing and non-performing categories. The fair value of loans is estimated using a discounted cash flow analysis. The discount rates used to determine fair value use interest rate spreads that reflect factors such as liquidity, credit, and non-performance risk of the loans. (f) Loans (Held-for-Sale) Held-for-sale loans are carried at the lower of aggregate cost or estimated fair value, less costs to sell, and therefore fair value is equal to carrying value. (g) Deposits The fair value of deposits with no stated maturity, such as interest and non-interest bearing demand deposits, savings, NOW and money market accounts, is equal to the amount payable on demand. The fair value of certificates of deposit is based on the discounted value of contractual cash flows. The discount rate is estimated using the rates currently offered for deposits of similar remaining maturities. (h) Commitments to Extend Credit and Standby Letters of Credit The fair value of commitments to extend credit and standby letters of credit is estimated using the fees currently charged to enter into similar agreements, taking into account the remaining terms of the agreements and the present creditworthiness of the counterparties. For fixed-rate loan commitments, fair value also considers the difference between current levels of interest rates and the committed rates. The fair value of off-balance sheet commitments is insignificant and therefore not included in the following table. (i) Borrowings The fair value of borrowed funds is estimated by discounting future cash flows based on rates currently available for debt with similar terms and remaining maturity. (j) Advance Payments by Borrowers for Taxes and Insurance Advance payments by borrowers for taxes and insurance have no stated maturity; the fair value is equal to the amount currently payable. (k) Derivatives The fair value of the Company's derivatives is determined using discounted cash flow analysis using observable market-based inputs, which are considered Level 2 inputs.
Earnings Per Share	Basic earnings per share is computed by dividing net income available to common stockholders by the weighted average number of shares outstanding during the period. For purposes of calculating basic earnings per share, weighted average common shares outstanding excludes unallocated employee stock ownership plan (“ESOP”) shares that have not been committed for release and unvested restricted stock and performance-based restricted stock units.Diluted earnings per share is computed using the same method as basic earnings per share, but reflects the potential dilution that could occur if stock options were exercised and converted into common stock and unvested shares of restricted stock and performance-based restricted stock units vested. These potentially dilutive shares are then included in the weighted average number of shares outstanding for the period using the treasury stock method. When applying the treasury stock method we added the assumed proceeds from option exercises and the average unamortized compensation costs related to unvested shares of restricted stock, performance-based restricted stock units and stock options. We then divided this sum by our average stock price for the period to calculate assumed shares repurchased. The excess of the number of shares issuable over the number of shares assumed to be repurchased is added to basic weighted average common shares to calculate diluted earnings per share.

Consolidated Financial Statements (Policies)

9 Months Ended

Sep. 30, 2022

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The consolidated financial statements are comprised of the accounts of Northfield Bancorp, Inc. and its wholly owned subsidiaries, Northfield Investments, Inc. and Northfield Bank (the “Bank”), and the Bank’s wholly-owned subsidiaries, NSB Services Corp. and NSB Realty Trust (collectively the “Company”). All significant intercompany accounts and transactions have been eliminated in consolidation.

In the opinion of management, all adjustments (consisting solely of normal and recurring adjustments) necessary for the fair presentation of the consolidated balance sheets and the consolidated statements of comprehensive income for the unaudited periods presented have been included. The results of operations and other data presented for the three and nine months ended September 30, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022 or for any other period. Whenever necessary, certain prior year amounts are reclassified to conform to the current year presentation.

In preparing the unaudited consolidated financial statements in conformity with U.S. generally accepted accounting principles (“U.S. GAAP”), management has made estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the consolidated balance sheets and for the periods indicated in the consolidated statements of comprehensive income. Material estimates that are particularly susceptible to change are: the allowance for credit losses, estimated cash flows of our purchased credit-deteriorated (“PCD”, or, previously, purchased credit-impaired “PCI”) loans and the valuation allowance against deferred tax assets. Estimates and assumptions are reviewed periodically and the effects of revisions are reflected in the consolidated financial statements in the period they are deemed necessary. While management uses its best judgment, actual amounts or results could differ significantly from those estimates.

Certain information and note disclosures usually included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for the preparation of interim financial statements. The consolidated financial statements presented should be read in conjunction with the audited consolidated financial statements and notes to consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the SEC.

Fair Value Measurements

The following tables present the assets reported on the consolidated balance sheets at their estimated fair value as of September 30, 2022, and December 31, 2021, by level within the fair value hierarchy as required by the Fair Value Measurements and Disclosures Topic of the FASB ASC. Financial assets and liabilities are classified in their entirety based on the level of input that is significant to the fair value measurement. The fair value hierarchy is as follows:

•Level 1 Inputs – Unadjusted quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

•Level 2 Inputs – Inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. These include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability (for example, interest rates, volatilities, prepayment speeds, loss severities, credit risks and default rates) or inputs that are derived principally from or corroborated by observable market data by correlations or other means.

•Level 3 Inputs – Significant unobservable inputs that reflect the Company’s own assumptions that market participants would use in pricing the assets or liabilities.

Fair Value of Financial Instruments

The FASB ASC Topic for Financial Instruments requires disclosure of the fair value of financial assets and financial liabilities, including those financial assets and financial liabilities that are not measured and reported at fair value on a recurring or non-recurring basis. The methodologies for estimating the fair value of financial assets and financial liabilities that are measured at fair value on a recurring or non-recurring basis are discussed above. The following methods and assumptions were used to estimate the fair value of other financial assets and financial liabilities not already discussed above:

(a) Cash and Cash Equivalents

Cash and cash equivalents are short-term in nature with original maturities of three months or less; the carrying amount approximates fair value. Certificates of deposit having original terms of six-months or less; the carrying value generally approximates fair value. Certificates of deposit with an original maturity of six months or greater; the fair value is derived from discounted cash flows.

(b) Debt Securities (Held-to-Maturity)

The estimated fair values for substantially all of our securities are obtained from an independent nationally recognized pricing service. The independent pricing service utilizes market prices of same or similar securities whenever such prices are available. Prices involving distressed sellers are not utilized in determining fair value. Where necessary, the independent third-party pricing service estimates fair value using models employing techniques such as discounted cash flow analysis. The assumptions used in these models typically include assumptions for interest rates, credit losses, and prepayments, utilizing market observable data where available.

The Company uses net asset value as a practical expedient to record its investment in a private SBA Loan Fund since the shares in the fund are not publicly traded, do not have a readily determinable fair value and the net asset value per share is calculated in a manner consistent with the measurement principles of an investment company.

(d) Federal Home Loan Bank of New York Stock

The fair value for Federal Home Loan Bank of New York ("FHLBNY") stock is its carrying value, since this is the amount for which it could be redeemed and there is no active market for this stock.

(e) Loans (Held-for-Investment)

Fair values are estimated for portfolios of loans with similar financial characteristics. Loans are segregated by type such as originated and purchased, and further segregated by residential mortgage, construction, land, multifamily, commercial and consumer. Each loan category is further segmented into amortizing and non-amortizing and fixed and adjustable rate interest terms and by performing and non-performing categories. The fair value of loans is estimated using a discounted cash flow analysis. The discount rates used to determine fair value use interest rate spreads that reflect factors such as liquidity, credit, and non-performance risk of the loans.

(f) Loans (Held-for-Sale)

Held-for-sale loans are carried at the lower of aggregate cost or estimated fair value, less costs to sell, and therefore fair value is equal to carrying value.

(g) Deposits

The fair value of deposits with no stated maturity, such as interest and non-interest bearing demand deposits, savings, NOW and money market accounts, is equal to the amount payable on demand. The fair value of certificates of deposit is based on the discounted value of contractual cash flows. The discount rate is estimated using the rates currently offered for deposits of similar remaining maturities.

(h) Commitments to Extend Credit and Standby Letters of Credit

The fair value of commitments to extend credit and standby letters of credit is estimated using the fees currently charged to enter into similar agreements, taking into account the remaining terms of the agreements and the present creditworthiness of the counterparties. For fixed-rate loan commitments, fair value also considers the difference between current levels of interest rates and the committed rates. The fair value of off-balance sheet commitments is insignificant and therefore not included in the following table.

(i) Borrowings

The fair value of borrowed funds is estimated by discounting future cash flows based on rates currently available for debt with similar terms and remaining maturity.

(j) Advance Payments by Borrowers for Taxes and Insurance

Advance payments by borrowers for taxes and insurance have no stated maturity; the fair value is equal to the amount currently payable.

(k) Derivatives

The fair value of the Company's derivatives is determined using discounted cash flow analysis using observable market-based inputs, which are considered Level 2 inputs.

Earnings Per Share

Basic earnings per share is computed by dividing net income available to common stockholders by the weighted average number of shares outstanding during the period. For purposes of calculating basic earnings per share, weighted average common shares outstanding excludes unallocated employee stock ownership plan (“ESOP”) shares that have not been committed for release and unvested restricted stock and performance-based restricted stock units.Diluted earnings per share is computed using the same method as basic earnings per share, but reflects the potential dilution that could occur if stock options were exercised and converted into common stock and unvested shares of restricted stock and performance-based restricted stock units vested. These potentially dilutive shares are then included in the weighted average number of shares outstanding for the period using the treasury stock method. When applying the treasury stock method we added the assumed proceeds from option exercises and the average unamortized compensation costs related to unvested shares of restricted stock, performance-based restricted stock units and stock options. We then divided this sum by our average stock price for the period to calculate assumed shares repurchased. The excess of the number of shares issuable over the number of shares assumed to be repurchased is added to basic weighted average common shares to calculate diluted earnings per share.