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

Commitments To Extend Credit, Allowance For Estimated Losses

No definition available.

Name:	nfbk_CommitmentsToExtendCreditAllowanceForEstimatedLosses
Namespace Prefix:	nfbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant