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v3.19.1

Loans - Narrative, Loan-to-Value Ratios (Details) | Mar. 31, 2019
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Commercial real estate |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Loan to value ratio, less than | 35.00%
One-to-four family residential mortgage |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Loan to value ratio, less than | 60.00%

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- 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

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- 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

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- 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: |