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Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract] |  |
Guarantees extend term period (years) | 1 year |
Allowance for estimated losses | $ 1,900 | $ 808
Agreements term (years) | 3 years |

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- Definition Agreement Term + References No definition available. + Details
Name: | nfbk_AgreementTerm
Namespace Prefix: | nfbk_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Commitments To Extend Credit, Allowance For Estimated Losses + References No definition available. + Details
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Namespace Prefix: | nfbk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Guaranteed Lease Residual Values Period + References No definition available. + Details
Name: | nfbk_GuaranteedLeaseResidualValuesPeriod
Namespace Prefix: | nfbk_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration