Loans - Amount of Loans Receivable Held-for-Investment and Related Allowances Per Loan Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Allowance for loan losses:
Beginning Balance	$ 28,707	$ 27,497
Charge-offs	(470)	(92)
Recoveries	380	22
Provisions (credit)	8,183	59
Ending balance	36,800	27,486
Ending balance: individually evaluated for impairment	62		$ 142
Ending balance: collectively evaluated for impairment	36,738		28,565
Loans, net:
Loans held-for-investment, net	3,509,137		3,437,085
Ending balance: individually evaluated for impairment	20,517		20,957
Ending balance: collectively evaluated for impairment	3,488,620		3,416,128
Originated
Allowance for loan losses:
Beginning Balance	27,783	26,487
Charge-offs	(37)	(32)
Recoveries	372	13
Provisions (credit)	7,838	8
Ending balance	35,956	26,476
Ending balance: individually evaluated for impairment	7		7
Ending balance: collectively evaluated for impairment	35,949		27,776
Loans, net:
Loans held-for-investment, net	3,098,760		2,987,067
Ending balance: individually evaluated for impairment	14,733		16,177
Ending balance: collectively evaluated for impairment	3,084,027		2,970,890
Purchased Credit-Impaired
Allowance for loan losses:
Beginning Balance	789	1,010
Charge-offs	0	0
Recoveries	0	0
Provisions (credit)	0	0
Ending balance	789	1,010
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	789		789
Loans, net:
Loans held-for-investment, net	16,886		17,365
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	16,886		17,365
Acquired Loans
Allowance for loan losses:
Beginning Balance	135	0
Charge-offs	(433)	(60)
Recoveries	8	9
Provisions (credit)	345	51
Ending balance	55	0
Ending balance: individually evaluated for impairment	55		135
Ending balance: collectively evaluated for impairment	0		0
Loans, net:
Loans held-for-investment, net	393,491		432,653
Ending balance: individually evaluated for impairment	5,784		4,780
Ending balance: collectively evaluated for impairment	387,707		427,873
Real Estate \| Commercial \| Originated
Allowance for loan losses:
Beginning Balance	4,756	5,630
Charge-offs	0	(6)
Recoveries	370	13
Provisions (credit)	263	(255)
Ending balance	5,389	5,382
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	5,389		4,756
Loans, net:
Loans held-for-investment, net	528,104		529,287
Ending balance: individually evaluated for impairment	11,815		13,226
Ending balance: collectively evaluated for impairment	516,289		516,061
Real Estate \| One-to-Four Family \| Originated
Allowance for loan losses:
Beginning Balance	180	342
Charge-offs	0	0
Recoveries	0	0
Provisions (credit)	132	28
Ending balance	312	370
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	312		180
Loans, net:
Loans held-for-investment, net	86,404		85,355
Ending balance: individually evaluated for impairment	1,816		1,841
Ending balance: collectively evaluated for impairment	84,588		83,514
Real Estate \| Construction and Land \| Originated
Allowance for loan losses:
Beginning Balance	536	463
Charge-offs	0	0
Recoveries	0	0
Provisions (credit)	135	111
Ending balance	671	574
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	671		536
Loans, net:
Loans held-for-investment, net	39,953		38,303
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	39,953		38,303
Real Estate \| Multifamily \| Originated
Allowance for loan losses:
Beginning Balance	20,203	18,084
Charge-offs	0	0
Recoveries	0	0
Provisions (credit)	6,686	262
Ending balance	26,889	18,346
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	26,889		20,203
Loans, net:
Loans held-for-investment, net	2,304,387		2,199,734
Ending balance: individually evaluated for impairment	994		997
Ending balance: collectively evaluated for impairment	2,303,393		2,198,737
Real Estate \| Home equity and lines of credit \| Originated
Allowance for loan losses:
Beginning Balance	317	291
Charge-offs	0	0
Recoveries	0	0
Provisions (credit)	257	45
Ending balance	574	336
Ending balance: individually evaluated for impairment	3		3
Ending balance: collectively evaluated for impairment	571		314
Loans, net:
Loans held-for-investment, net	89,349		86,848
Ending balance: individually evaluated for impairment	53		55
Ending balance: collectively evaluated for impairment	89,296		86,793
Commercial and Industrial \| Originated
Allowance for loan losses:
Beginning Balance	1,640	1,569
Charge-offs	(37)	(26)
Recoveries	2	0
Provisions (credit)	430	(167)
Ending balance	2,035	1,376
Ending balance: individually evaluated for impairment	4		4
Ending balance: collectively evaluated for impairment	2,031		1,636
Loans, net:
Loans held-for-investment, net	49,358		45,456
Ending balance: individually evaluated for impairment	55		58
Ending balance: collectively evaluated for impairment	49,303		45,398
Other \| Originated
Allowance for loan losses:
Beginning Balance	151	108
Charge-offs	0	0
Recoveries	0	0
Provisions (credit)	(65)	(16)
Ending balance	86	$ 92
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	86		151
Loans, net:
Loans held-for-investment, net	1,205		2,084
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	$ 1,205		$ 2,084

Originated loans, net.

No definition available.

Name:	nfbk_OriginatedLoansNet
Namespace Prefix:	nfbk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

+ References

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

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(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
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=118936363&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=118939070&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=118936363&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

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=118952595&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=118936363&loc=d3e5074-111524

+ Details

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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

+ Details

Name:	us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
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:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nfbk_RealEstatePortfolioSegmentMember
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=nfbk_ConstructionAndLandLoansMember
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_HomeEquityMember
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: