Deferred loan cost, net.

No definition available.

Name:	nfbk_DeferredLoanCostNet
Namespace Prefix:	nfbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Loans receivable commercial and industrial insurance premium and other loans.

+ References

No definition available.

+ Details

Name:	nfbk_LoansReceivableCommercialAndIndustrialInsurancePremiumAndOtherLoans
Namespace Prefix:	nfbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Purchased credit-impaired (PCI) loans held-for-investment

+ References

No definition available.

+ Details

Name:	nfbk_PurchasedCreditImpairedLoansHeldForInvestment
Namespace Prefix:	nfbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansAndLeasesReceivableDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetReportedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetOriginatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nfbk_RealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nfbk_OneToFourFamilyResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nfbk_ConstructionAndLandLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nfbk_CommercialAndIndustrialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nfbk_OtherPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredAndNoCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans - Net Loans Held-for-Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Loans held-for-investment, net	$ 3,437,085	$ 3,245,170
PCI Loans	17,365	20,143
Allowance for loan losses	(28,707)	(27,497)	$ (26,160)	$ (24,595)
Net loans held-for-investment	3,408,378	3,217,673
Originated loans
Loans and Leases Receivable Disclosure [Line Items]
Total commercial and industrial and other loans	47,411	45,623
Deferred loan cost, net	7,614	6,892
Loans held-for-investment, net	2,987,067	2,678,877
Allowance for loan losses	(27,783)	(26,487)	(25,172)
Originated loans \| Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,932,042	2,626,362
Originated loans \| Real Estate \| Multifamily
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,196,407	1,930,535
Loans held-for-investment, net	2,199,734	1,933,946
Allowance for loan losses	(20,203)	(18,084)	(17,374)
Originated loans \| Real Estate \| Commercial mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	528,681	499,311
Loans held-for-investment, net	529,287	499,860
Allowance for loan losses	(4,756)	(5,630)	(5,196)
Originated loans \| Real Estate \| One-to-four family residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	83,742	91,371
Loans held-for-investment, net	85,355	92,433
Allowance for loan losses	(180)	(342)	(503)
Originated loans \| Real Estate \| Home equity and lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans	84,928	78,593
Loans held-for-investment, net	86,848	80,315
Allowance for loan losses	(317)	(291)	(122)
Originated loans \| Real Estate \| Construction and land
Loans and Leases Receivable Disclosure [Line Items]
Loans	38,284	26,552
Loans held-for-investment, net	38,303	26,613
Allowance for loan losses	(536)	(463)	(610)
Originated loans \| Commercial and Industrial
Loans and Leases Receivable Disclosure [Line Items]
Loans	45,328	44,104
Loans held-for-investment, net	45,456	44,190
Allowance for loan losses	(1,640)	(1,569)	(1,273)
Originated loans \| Other
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,083	1,519
Loans held-for-investment, net	2,084	1,520
Allowance for loan losses	(151)	(108)	(94)
Acquired Loans
Loans and Leases Receivable Disclosure [Line Items]
Loans	432,653	546,150
Loans held-for-investment, net	432,653	546,150
Allowance for loan losses	(135)	0	$ (37)
Acquired Loans \| Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans	423,964	534,211
Acquired Loans \| Real Estate \| Multifamily
Loans and Leases Receivable Disclosure [Line Items]
Loans	108,417	145,485
Acquired Loans \| Real Estate \| Commercial mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	113,027	133,263
Acquired Loans \| Real Estate \| One-to-four family residential mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	187,975	225,877
Acquired Loans \| Real Estate \| Home equity and lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans	12,008	17,583
Acquired Loans \| Real Estate \| Construction and land
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,537	12,003
Acquired Loans \| Commercial and Industrial
Loans and Leases Receivable Disclosure [Line Items]
Loans	8,689	11,933
Acquired Loans \| Other
Loans and Leases Receivable Disclosure [Line Items]
Loans	$ 0	$ 6