| Cover Page - USD ($) $ in Millions | 12 Months Ended | ||
| Dec. 31, 2020 | Feb. 28, 2021 | Jun. 30, 2020 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Document Annual Report | true | ||
| Document Period End Date | Dec. 31, 2020 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Document Transition Report | false | ||
| Entity File Number | 001-35791 | ||
| Entity Registrant Name | Northfield Bancorp, Inc. | ||
| Entity Incorporation, State or Country Code | DE | ||
| Entity Tax Identification Number | 80-0882592 | ||
| Entity Address, Address Line One | 581 Main Street, | ||
| Entity Address, City or Town | Woodbridge, | ||
| Entity Address, State or Province | NJ | ||
| Entity Address, Postal Zip Code | 07095 | ||
| City Area Code | 732 | ||
| Local Phone Number | 499-7200 | ||
| Title of 12(b) Security | Common Stock, par value $0.01 per share | ||
| Trading Symbol | NFBK | ||
| Security Exchange Name | NASDAQ | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Voluntary Filers | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Filer Category | Accelerated Filer | ||
| Entity Small Business | false | ||
| Entity Emerging Growth Company | false | ||
| ICFR Auditor Attestation Flag | true | ||
| Entity Shell Company | false | ||
| Entity Public Float | $ 509.8 | ||
| Entity Common Stock, Shares Outstanding | 51,763,101 | ||
| Documents Incorporated by Reference | None. | ||
| Amendment Flag | false | ||
| Document Fiscal Year Focus | 2020 | ||
| Document Fiscal Period Focus | FY | ||
| Entity Central Index Key | 0001493225 |
