Summary Of Significant Accounting Policies (Policy)	12 Months Ended
Summary Of Significant Accounting Policies (Policy)	Dec. 31, 2013
Summary Of Significant Accounting Policies [Abstract]
Plan Of Conversion And Reorganization	(a) Plan of Conversion and Reorganization On June 6, 2012, the Board of Trustees of Northfield Bancorp, MHC (MHC) and the Board of Directors of the Company adopted a Plan of Conversion and Reorganization (the Plan). Pursuant to the Plan, the MHC converted from the mutual holding company form of organization to the fully public form on January 24, 2013. The MHC was merged into the Company, and the MHC no longer exists. The Company merged into a new Delaware corporation named Northfield Bancorp, Inc. As part of the conversion, the MHC’s ownership interest of the Company was offered for sale in a public offering. The existing publicly held shares of the Company, which represented the remaining ownership interest in the Company, were exchanged for new shares of common stock of Northfield Bancorp, Inc., the new Delaware Corporation. The exchange ratio ensured that immediately after the conversion and public offering, the public shareholders of the Company owned the same aggregate percentage of Northfield Bancorp., Inc. common stock that they owned immediately prior to that time (excluding shares purchased in the stock offering and cash received in lieu of fractional shares). When the conversion and public offering was completed, all of the capital stock of Northfield Bank was owned by Northfield Bancorp, Inc., the Delaware Corporation. The Plan provided for the establishment of special “liquidation accounts” for the benefit of certain depositors of Northfield Bank in an amount equal to the greater of the MHC’s ownership interest in the retained earnings of the Company as of the date of the latest balance sheet contained in the prospectus or the retained earnings of Northfield Bank at the time it reorganized into the MHC. Following the completion of the conversion, under the rules of the Board of Governors of the Federal Reserve System, Northfield Bank is not permitted to pay dividends on its capital stock to Northfield Bancorp., Inc., its sole shareholder, if Northfield Bank’s shareholder’s equity would be reduced below the amount of the liquidation accounts. The liquidation accounts will be reduced annually to the extent that eligible account holders have reduced their qualifying deposits. Subsequent increases in qualifying deposits will not restore an eligible account holder’s interest in the liquidation accounts. Direct costs of the conversion and public offering were deferred and reduced the proceeds from the shares sold in the public offering. Costs of $11.5 million were incurred related to the conversion. Share and per share amounts have been restated to reflect the completion of our second-step conversion on January 24, 2013 at a conversion ratio of 1.4029 unless noted otherwise.
Basis Of Presentation	(b) Basis of Presentation The consolidated financial statements are comprised of the accounts of Northfield Bancorp, Inc. and its wholly owned subsidiaries, Northfield Investment, Inc. and Northfield Bank (the Bank) and the Bank’s wholly-owned significant subsidiaries, NSB Services Corp. and NSB Realty Trust. All significant intercompany accounts and transactions have been eliminated in consolidation. In preparing the consolidated financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheets and revenues and expenses during the reporting periods. Actual results may differ significantly from those estimates and assumptions. A material estimate that is particularly susceptible to significant change in the near term is the allowance for loan losses. In connection with the determination of this allowance, management generally obtains independent appraisals for significant properties. In addition, judgments related to the amount and timing of expected cash flows from purchased credit-impaired loans, goodwill, securities valuation and impairment, and deferred income taxes, involve a higher degree of complexity and subjectivity and require estimates and assumptions about highly uncertain matters. Actual results may differ from the estimates and assumptions. Certain prior year balances have been reclassified to conform to the current year presentation.
Business	(c) Business The Company, through its principal subsidiary, the Bank, provides a full range of banking services primarily to individuals and corporate customers in Richmond and Kings Counties in New York, and Union and Middlesex Counties in New Jersey. The Company is subject to competition from other financial institutions and to the regulations of certain federal and state agencies, and undergoes periodic examinations by those regulatory authorities.
Cash Equivalents	(d) Cash Equivalents Cash equivalents consist of cash on hand, due from banks, and interest-bearing deposits in other financial institutions with an original term of three months or less.
Securities	(e) Securities Securities are classified at the time of purchase, based on management’s intention, as securities held- to-maturity, securities available-for-sale, or trading account securities. Securities held-to-maturity are those that management has the positive intent and ability to hold until maturity. Securities held-to-maturity are carried at amortized cost, adjusted for amortization of premiums and accretion of discounts using the level‑yield method over the contractual term of the securities, adjusted for actual prepayments. Securities available-for-sale represents all securities not classified as either held-to-maturity or trading. Securities available-for-sale are carried at estimated fair value with unrealized holding gains and losses (net of related tax effects) on such securities excluded from earnings, but included as a separate component of stockholders’ equity, titled “Accumulated other comprehensive income (loss).” The cost of securities sold is determined using the specific‑identification method. Security transactions are recorded on a trade-date basis. Trading securities are securities that are bought and may be held for the purpose of selling them in the near term. Trading securities are reported at estimated fair value, with unrealized holding gains and losses reported as a component of gain (loss) on securities transactions, net in non-interest income. Our evaluation of other-than-temporary impairment considers the duration and severity of the impairment, our intent and ability to hold the securities, and our assessments of the reason for the decline in value and the likelihood of a near-term recovery. If a determination is made that a debt security is other-than-temporarily impaired, the Company will estimate the amount of the unrealized loss that is attributable to credit and all other non-credit related factors. If we intend to hold securities in an unrealized loss position until the loss is recovered, which may be at maturity, the credit related component will be recognized as an other-than-temporary impairment charge in non-interest income. The non-credit related component will be recorded as an adjustment to accumulated other comprehensive income (loss), net of tax. The estimated fair value of debt securities, including mortgage-backed securities and corporate debt obligations is furnished by an independent third-party pricing service. The third-party pricing service primarily utilizes pricing models and methodologies that incorporate observable market inputs, including among other things, benchmark yields, reported trades, and projected prepayment and default rates. Management reviews the data and assumptions used in pricing the securities by its third-party provider for reasonableness.
Loans	(f) Loans During 2012 and 2011, the Company acquired loans with deteriorated credit quality, herein referred to as purchased credit-impaired (PCI) loans, and transferred certain loans, previously originated and designated by the Company as held-for-investment, to held-for-sale. The accounting and reporting for these loans differs substantially from those loans originated and classified by the Company as held-for-investment. For purposes of reporting, discussion and analysis, management has classified its loan portfolio into four categories: (1) loans originated by the Company and held-for-sale, which are carried at the lower of aggregate cost or estimated fair value, less costs to sell, and therefore have no associated allowance for loan losses, (2) PCI loans, which are held-for-investment, and initially valued at estimated fair value on the date of acquisition, with no initial related allowance for loan losses, and (3) originated loans held-for-investment, which are carried at amortized cost, less net charge-offs and the allowance for loan losses, and (4) acquired loans with no evidence of credit deterioration, which are held-for-investment, and initially valued at an estimated fair value on the date of acquisition, with no initial related allowance for loan losses. Originated and acquired net loans held-for-investment are stated at unpaid principal balance, adjusted by unamortized premiums and unearned discounts, deferred origination fees and certain direct origination costs, and the allowance for loan losses. Interest income on loans is accrued and credited to income as earned. Net loan origination fees/costs are deferred and accreted/amortized to interest income over the loan’s contractual life using the level-yield method, adjusted for actual prepayments. Generally, loans held-for-sale are designated at time of origination and generally consist of newly originated fixed rate residential loans and are recorded at the lower of aggregate cost or estimated fair value in the aggregate. In 2013 and 2012, the Company transferred from held-for-investment to held-for-sale certain impaired loans. Transfers from held-for-investment are infrequent and occur at fair value less costs to sell, with any charge-off to allowance for loan losses. Gains are recognized on a settlement-date basis and are determined by the difference between the net sales proceeds and the carrying value of the loans, including any net deferred fees or costs. Originated and acquired net loans held-for-investment are deemed impaired when it is probable, based on current information, that the Company will not collect all amounts due in accordance with the contractual terms of the loan agreement. The Company has defined the population of originated and acquired impaired loans to be all originated and acquired non-accrual loans held-for-investment with an outstanding balance of $500,000 or greater and all loans restructured in troubled debt restructurings (TDRs). Originated and acquired impaired loans held-for-investment are individually assessed to determine that the loan’s carrying value is not in excess of the expected future cash flows, discounted at the loans original effective interest rate, or the underlying collateral (less estimated costs to sell) if the loan is collateral dependent. Impairments, if any, are recognized through a charge to the provision for loan losses for the amount that the loan’s carrying value exceeds the discounted cash flow analysis or estimated fair value of collateral (less estimated costs to sell) if the loan is collateral dependent. Such amounts are charged-off when considered appropriate. Homogeneous loans with balances less than $500,000, which are not considered TDRs, are collectively evaluated for impairment. The allowance for loan losses is increased by the provision for loan losses charged against income and is decreased by charge-offs, net of recoveries. Loan losses are charged-off in the period the loans, or portion thereof, are deemed uncollectible. Generally, the Company will record a loan charge-off (including a partial charge-off) to reduce a loan to the estimated fair value of the underlying collateral, less cost to sell, if it is determined that it is probable that recovery will come primarily from the sale or operation of such collateral. Specific reserves on impaired loans which are not considered collateral dependent are charged-off when such amounts are not considered to be collectible. The provision for loan losses is based on management’s evaluation of the adequacy of the allowance which considers, among other things, impaired loans held-for-investment, deterioration in PCI loans subsequent to acquisition, past loan loss experience, known and inherent risks in the portfolio, and existing adverse situations that may affect borrowers’ ability to repay. Additionally, management evaluates changes, if any, in underwriting standards, collection, charge-off and recovery practices, the nature or volume of the portfolio, lending staff, concentration of loans, as well as current economic conditions, and other relevant factors. Management believes the allowance for loan losses is adequate to provide for probable and reasonably estimable losses at the date of the consolidated balance sheets. The Company also maintains an allowance for estimated losses on off-balance sheet credit risks related to loan commitments and standby letters of credit. Management utilizes a methodology similar to its allowance for loan loss adequacy methodology to estimate losses on these commitments. The allowance for estimated credit losses on off-balance sheet commitments is included in other liabilities and any changes to the allowance are recorded as a component of other non-interest expense. While management uses available information to recognize probable and reasonably estimable losses on loans, future additions may be necessary based on changes in conditions, including changes in economic conditions, particularly in Richmond and Kings Counties in New York, and Union and Middlesex Counties in New Jersey. Accordingly, as with most financial institutions in the market area, the ultimate collectability of a substantial portion of the Company’s loan portfolio is susceptible to changes in conditions in the Company’s marketplace. In addition, future changes in laws and regulations could make it more difficult for the Company to collect all contractual amounts due on its loans and mortgage-backed securities. In addition, various regulatory agencies, as an integral part of their examination process, periodically review the Company’s allowance for loan losses. Such agencies may require the Company to recognize additions to the allowance based on their judgments about information available to them at the time of their examination. Troubled debt restructured loans are those loans whose terms have been modified because of deterioration in the financial condition of the borrower. Modifications could include extension of the repayment terms of the loan, reduced interest rates, and forgiveness of accrued interest and/or principal. Once an obligation has been restructured because of such credit problems, it continues to be considered restructured until paid in full or, if the obligation yields a market rate (a rate equal to the rate the Company was willing to accept at the time of the restructuring for a new loan with comparable risk), until the year subsequent to the year in which the restructuring takes place, provided the borrower has performed under the modified terms for a six-month period. The Company records an impairment charge equal to the difference between the present value of estimated future cash flows under the restructured terms discounted at the original loan’s effective interest rate, or the underlying collateral value less costs to sell, if the loan is collateral dependent. Changes in present values attributable to the passage of time are recorded as a component of the provision for loan losses. A loan is considered past due when it is not paid in accordance with its contractual terms. The accrual of income on loans, including impaired loans held-for-investment, and other loans in the process of foreclosure, is generally discontinued when a loan becomes 90 days or more delinquent, or sooner when certain factors indicate that the ultimate collection of principal and interest is in doubt. Loans on which the accrual of income has been discontinued are designated as non-accrual loans. All previously accrued interest is reversed against interest income, and income is recognized subsequently only in the period that cash is received, provided no principal payments are due and the remaining principal balance outstanding is deemed collectible. A non-accrual loan is not returned to accrual status until both principal and interest payments are brought current and factors indicating doubtful collection no longer exist, including performance by the borrower under the loan terms for a six-month period. The Company accounts for the PCI loans based on expected cash flows. This election is in accordance with FASB Accounting Standards Codification (ASC) Topic 310-30, “Loans and Debt Securities Acquired with Deteriorated Credit Quality” (ASC 310-30). In accordance with ASC 310-30, the Company will maintain the integrity of a pool of multiple loans accounted for as a single asset and evaluate the pools for impairment, and accrual status, based on variances from the expected cash flows.
Federal Home Loan Bank Stock	(g) Federal Home Loan Bank Stock The Bank, as a member of the Federal Home Loan Bank of New York (the FHLB), is required to hold shares of capital stock in the FHLB as a condition to both becoming a member and engaging in certain transactions with the FHLB. The minimum investment requirement is determined by a “membership” investment component and an “activity-based” investment component. The membership investment component is the greater of 0.20% of the Bank’s mortgage-related assets, as defined by the FHLB, or $1,000. The activity-based investment component is equal to 4.5% of the Bank’s outstanding advances with the FHLB. The activity-based investment component also considers other transactions, including assets originated for or sold to the FHLB, and delivery commitments issued by the FHLB. The Company currently does not enter into these other types of transactions with the FHLB. On a quarterly basis, we perform our other-than-temporary impairment analysis of FHLB stock, we evaluate, among other things, (i) its earnings performance, including the significance of any decline in net assets of the FHLB as compared to the regulatory capital amount of the FHLB, (ii) the commitment by the FHLB to continue dividend payments, and (iii) the liquidity position of the FHLB. We did not consider our investment in FHLB stock to be other-than-temporarily impaired at December 31, 2013, and 2012.
Premises And Equipment, Net	(h) Premises and Equipment, Net Premises and equipment, including leasehold improvements, are carried at cost, less accumulated depreciation and amortization. Depreciation and amortization of premises and equipment, including capital leases, are computed on a straight‑line basis over the estimated useful lives of the related assets. The estimated useful lives of significant classes of assets are generally as follows: buildings - forty years; furniture and equipment - five to seven years; and purchased computer software - three years. Leasehold improvements are amortized over the shorter of the term of the related lease or the estimated useful lives of the improvements. Major improvements are capitalized, while repairs and maintenance costs are charged to operations as incurred. Upon retirement or sale, any gain or loss is credited or charged to operations.
Bank Owned Life Insurance	(i) Bank Owned Life Insurance The Company has purchased bank owned life insurance contracts to help fund its obligations for certain employee benefit costs. The Company’s investment in such insurance contracts has been reported in the consolidated balance sheets at their cash surrender values. Changes in cash surrender values and death benefit proceeds received in excess of the related cash surrender values are recorded as non-interest income.
Goodwill	(j) Goodwill Intangible assets resulting from acquisitions under the purchase method of accounting consist of goodwill and other intangible assets. Goodwill is not amortized and is subject to an annual assessment for impairment. The goodwill impairment analysis is generally a two-step test. However, we may, under Accounting Standards Update (ASU) No. 2011-08, “Testing Goodwill for Impairment,” first assess qualitative factors to determine whether it is necessary to perform the two-step quantitative goodwill impairment test. Under this new ASU, we are not required to calculate the fair value of a reporting unit if, based on a qualitative assessment, we determine that it was more likely than not that the unit’s fair value was not less than its carrying amount. During 2013, we elected to perform step one of the two-step goodwill impairment test for our reporting unit, but (under the ASU) we may choose to perform the optional qualitative assessment in future periods. Goodwill is allocated to Northfield’s reporting unit at the date goodwill is actually recorded. If the carrying value of a reporting unit exceeds its estimated fair value, a second step in the analysis is performed to determine the amount of impairment, if any. The second step compares the implied fair value of the reporting unit’s goodwill with the carrying amount of that goodwill. If the carrying value of a reporting unit exceeds the implied fair value of the goodwill, an impairment charge is recorded equal to the excess amount in the current period earnings. As of December 31, 2013, the carrying value of goodwill totaled $16.2 million. The Company performed its annual goodwill impairment test, as of December 31, 2013, and determined that the fair value of the Company’s single reporting unit to be in excess of its carrying value. The Company will test goodwill for impairment between annual test dates if an event occurs or circumstances change that would indicate the fair value of the reporting unit is below its carrying amount. No events have occurred and no circumstances have changed since the annual impairment test date that would indicate the fair value of the reporting unit is below its carrying amount.
Income Taxes	(k) Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply in the year in which those temporary differences are expected to be recovered or settled. When applicable, deferred tax assets are reduced by a valuation allowance for any portions determined not likely to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Income tax benefits are recognized and measured based upon a two-step model: 1) a tax position must be more-likely-than-not to be sustained based solely on its technical merits in order to be recognized, and 2) the benefit is measured as the largest dollar amount of that position that is more-likely-than-not to be sustained upon settlement. The difference between the benefit recognized and the tax benefit claimed on a tax return is referred to as an unrecognized tax benefit (UTB). The Corporation records income tax-related interest and penalties, if applicable, within income tax expense.
Impairment Of Long-Lived Assets	(l) Impairment of Long‑Lived Assets Long‑lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to future undiscounted (and without interest) net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the fair value of the assets. Assets to be disposed of are reported at the lower of the carrying amount or fair value less costs to sell.
Securities Sold Under Agreements To Repurchase And Other Borrowings	(m) Securities Sold Under Agreements to Repurchase and Other Borrowings The Company enters into sales of securities under agreements to repurchase (Repurchase Agreements) and collateral pledge agreements (Pledge Agreements) with selected dealers and banks. Such agreements are accounted for as secured financing transactions since the Company maintains effective control over the transferred or pledged securities and the transfer meets the other accounting and recognition criteria as required by the transfer and servicing topic of the FASB Accounting Standards. Obligations under these agreements are reflected as a liability in the consolidated balance sheets. Securities underlying the agreements are maintained at selected dealers and banks as collateral for each transaction executed and may be sold or pledged by the counterparty. Collateral underlying Repurchase Agreements which permit the counterparty to sell or pledge the underlying collateral is disclosed on the consolidated balance sheets as “encumbered.” The Company retains the right under all Repurchase Agreements and Pledge Agreements to substitute acceptable collateral throughout the terms of the agreement.
Comprehensive (Loss) Income	(n) Comprehensive (Loss) Income Comprehensive (loss) income includes net income and the change in unrealized holding gains and losses on securities available-for-sale, change in actuarial gains and losses on other post retirement benefits, and change in service cost on other postretirement benefits, net of taxes. Comprehensive (loss) income is presented in the Consolidated Statements of Comprehensive (Loss) Income.
Employee Benefits	(o) Employee Benefits The Company sponsors a defined postretirement benefit plan that provides for medical and life insurance coverage to a limited number of retirees, as well as life insurance to all qualifying employees of the Company. The estimated cost of postretirement benefits earned is accrued during an individual’s estimated service period to the Company. The Company recognizes in its balance sheet the over-funded or under-funded status of a defined benefit postretirement plan measured as the difference between the fair value of plan assets and the benefit obligation at the end of our calendar year. The actuarial gains and losses and the prior service costs and credits that arise during the period are recognized as a component of other comprehensive (loss) income, net of tax. Funds borrowed by the Employee Stock Ownership Plan (ESOP) from the Company to purchase the Company’s common stock are being repaid from the Bank’s contributions over a period of up to 30 years. The Company’s common stock not yet allocated to participants is recorded as a reduction of stockholders’ equity at cost. The Company records compensation expense related to the ESOP at an amount equal to the shares committed to be released by the ESOP multiplied by the average fair value of our common stock during the reporting period. The Company recognizes the grant-date fair value of stock based awards issued to employees as compensation cost in the consolidated statements of comprehensive (loss) income. The fair value of common stock awards is based on the closing price of our common stock as reported on the NASDAQ Stock Market on the grant date. The expense related to stock options is based on the estimated fair value of the options at the date of the grant using the Black-Scholes pricing model. The awards are fixed in nature and compensation cost related to stock based awards is recognized on a straight-line basis over the requisite service periods. The Bank has a 401(k) plan covering substantially all employees. Contributions to the plan are expensed as incurred.
Segment Reporting	(p) Segment Reporting As a community-focused financial institution, substantially all of the Company’s operations involve the delivery of loan and deposit products to customers. Management makes operating decisions and assesses performance based on an ongoing review of these community banking operations, which constitute the Company’s only operating segment for financial reporting purposes.
Net Income Per Common Share	(q) Net Income per Common Share Net income per common share-basic is computed by dividing the net income available to common stockholders by the weighted average number of common shares outstanding, excluding unallocated ESOP shares and unearned common stock award shares. The weighted average common shares outstanding includes the average number of shares of common stock outstanding, including shares held by Northfield Bancorp, MHC and allocated or committed to be released ESOP shares. Net income per common share-diluted is computed using the same method as basic earnings per share, but reflects the potential dilution that could occur if stock options and unvested shares of restricted stock were exercised and converted into common stock. These potentially dilutive shares are included in the weighted average number of shares outstanding for the period using the treasury stock method. When applying the treasury stock method, we add: (1) the assumed proceeds from option exercises; (2) the tax benefit, if any, that would have been credited to additional paid-in capital assuming exercise of non-qualified stock options and vesting of shares of restricted stock; and (3) the average unamortized compensation costs related to unvested shares of restricted stock and stock options. We then divide 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. At December 31, 2013, 2012, and 2011, there were 922,629, 776,213, and 626,094 dilutive shares outstanding, respectively.
Other Real Estate Owned	(r) Other Real Estate Owned Assets acquired through loan foreclosure, or deed-in-lieu of, are held for sale and are initially recorded at estimated fair value less estimated selling costs when acquired, thus establishing a new cost basis. Costs after acquisition are generally expensed. If the estimated fair value of the asset declines, a write-down is recorded through other non-interest expense.