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Loans - Narrative, Loan-to-Value Ratios (Details)	Jun. 30, 2020
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan to value ratio, less than	35.00%
One-to-Four Family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan to value ratio, less than	60.00%

X

- Definition

Loan to value ratio.

+ References

No definition available.

+ Details

Name:	nfbk_LoanToValueRatio
Namespace Prefix:	nfbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

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

X

- Details

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

X

- Details

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