| Cover Page Cover Page | Jul. 29, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 1, 2021 |
| Entity Registrant Name | Columbia Financial, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38456 |
| Entity Tax Identification Number | 22-3504946 |
| Entity Address, Address Line One | 19-01 Route 208 North |
| Entity Address, City or Town | Fair Lawn |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 07410 |
| City Area Code | 800 |
| Local Phone Number | 522-4167 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, $0.01 par value per share |
| Trading Symbol | CLBK |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001723596 |
