Loans - Summary of Impaired Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Recorded Investment
With No Allowance Recorded	$ 12,900	$ 15,900
Total	19,881	20,957
Unpaid Principal Balance
Total	22,304	23,132
Related Allowance	(319)	(142)
Real Estate \| Commercial
Recorded Investment
Total	16,785	17,327
Unpaid Principal Balance
Total	18,614	18,916
Related Allowance	(307)	(135)
Real Estate \| One-to-Four Family
Recorded Investment
Total	2,318	2,479
Unpaid Principal Balance
Total	2,441	2,595
Related Allowance	(4)	0
Real Estate \| Multifamily
Recorded Investment
Total	673	1,038
Unpaid Principal Balance
Total	1,144	1,508
Related Allowance	0	0
Real Estate \| Home Equity and Lines of Credit
Recorded Investment
Total	51	55
Unpaid Principal Balance
Total	51	55
Related Allowance	(4)	(3)
Real Estate \| Home Equity and Lines of Credit \| Substandard
Recorded Investment
With a Related Allowance Recorded	32	33
Unpaid Principal Balance
With a Related Allowance Recorded	32	33
Related Allowance	(4)	(3)
Real Estate \| Home Equity and Lines of Credit \| Pass
Recorded Investment
With No Allowance Recorded	19	22
Unpaid Principal Balance
With No Allowance Recorded	19	22
Commercial and Industrial
Recorded Investment
Total	54	58
Unpaid Principal Balance
Total	54	58
Related Allowance	(4)	(4)
Commercial and Industrial \| Substandard
Recorded Investment
With No Allowance Recorded	37	39
Unpaid Principal Balance
With No Allowance Recorded	37	39
Commercial and Industrial \| Special Mention
Recorded Investment
With a Related Allowance Recorded	17	19
Unpaid Principal Balance
With a Related Allowance Recorded	17	19
Related Allowance	(4)	(4)
Less Than 35% LTV \| Real Estate \| Commercial \| Substandard
Recorded Investment
With No Allowance Recorded	79	0
Unpaid Principal Balance
With No Allowance Recorded	218	139
Less Than 35% LTV \| Real Estate \| Multifamily \| Substandard
Recorded Investment
With No Allowance Recorded	40	40
Unpaid Principal Balance
With No Allowance Recorded	40	40
Equal to or Greater than 35% LTV \| Real Estate \| Commercial \| Substandard
Recorded Investment
With No Allowance Recorded	8,449	10,438
With a Related Allowance Recorded	2,799	1,307
Unpaid Principal Balance
With No Allowance Recorded	8,655	11,002
With a Related Allowance Recorded	3,397	1,307
Related Allowance	(288)	(135)
Equal to or Greater than 35% LTV \| Real Estate \| Commercial \| Pass
Recorded Investment
With No Allowance Recorded	3,629	5,582
With a Related Allowance Recorded	1,318	0
Unpaid Principal Balance
With No Allowance Recorded	4,378	6,468
With a Related Allowance Recorded	1,318	0
Related Allowance	(19)	0
Equal to or Greater than 35% LTV \| Real Estate \| Commercial \| Special Mention
Recorded Investment
With No Allowance Recorded	511	0
Unpaid Principal Balance
With No Allowance Recorded	648	0
Equal to or Greater than 35% LTV \| Real Estate \| Multifamily \| Substandard
Recorded Investment
With No Allowance Recorded	611	972
Unpaid Principal Balance
With No Allowance Recorded	611	972
Equal to or Greater than 35% LTV \| Real Estate \| Multifamily \| Pass
Recorded Investment
With No Allowance Recorded	0	26
Unpaid Principal Balance
With No Allowance Recorded	0	496
Equal to or Greater than 35% LTV \| Real Estate \| Multifamily \| Special Mention
Recorded Investment
With No Allowance Recorded	22	0
Unpaid Principal Balance
With No Allowance Recorded	493	0
Less Than 60% LTV \| Real Estate \| One-to-Four Family \| Substandard
Recorded Investment
With No Allowance Recorded	218	564
With a Related Allowance Recorded	522	0
Unpaid Principal Balance
With No Allowance Recorded	218	564
With a Related Allowance Recorded	522	0
Related Allowance	(4)	0
Less Than 60% LTV \| Real Estate \| One-to-Four Family \| Pass
Recorded Investment
With No Allowance Recorded	1,057	1,379
Unpaid Principal Balance
With No Allowance Recorded	1,145	1,463
Less Than 60% LTV \| Real Estate \| One-to-Four Family \| Special Mention
Recorded Investment
With No Allowance Recorded	378	385
Unpaid Principal Balance
With No Allowance Recorded	378	385
Equal to or Greater than 60% LTV \| Real Estate \| One-to-Four Family \| Substandard
Recorded Investment
With No Allowance Recorded	25	29
Unpaid Principal Balance
With No Allowance Recorded	25	29
Equal to or Greater than 60% LTV \| Real Estate \| One-to-Four Family \| Pass
Recorded Investment
With No Allowance Recorded	118	122
Unpaid Principal Balance
With No Allowance Recorded	$ 153	$ 154

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953550-111524

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

- 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=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_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
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_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=nfbk_LessThan35LtvMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=nfbk_EqualToOrGreaterThan35LtvMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=nfbk_LessThan60LtvMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanToValueAxis=nfbk_EqualToOrGreaterThan60LtvMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: