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Loans - Narrative, Collateral-Dependent Loans (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Financing Receivable, Allowance for Credit Losses [Line Items] |  |
Collateral-dependent impaired loans | $ 7,400 | $ 10,200
Real Estate | Commercial |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |
Collateral-dependent impaired loans | 5,900 |
Real Estate | Multifamily |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |
Collateral-dependent impaired loans | 1,100 |
Real Estate | One-to-Four Family |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |
Collateral-dependent impaired loans | $ 365 |

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- Definition Financing Receivable, Impaired, Secured By Collateral + References No definition available. + Details
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Namespace Prefix: | nfbk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
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Data Type: | xbrli:stringItemType
Balance Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |