v3.26.1
Cover
3 Months Ended
Mar. 31, 2026
shares
Cover [Abstract]  
Document Type 10-Q
Document Quarterly Report true
Document Transition Report false
Entity File Number 333-294103
Registrant Name Columbia Financial, Inc.
Entity Incorporation, State or Country Code MD
Entity Tax Identification Number 42-1991301
Entity Address, Address Line One 19-01 Route 208 North
Entity Address, Postal Zip Code 07410
Entity Address, City or Town Fair Lawn
Entity Address, State or Province NJ
City Area Code 800
Local Phone Number 522-4167
Title of 12(b) Security None
Trading Symbol
Security Exchange Name
Entity Current Reporting Status No
Entity Interactive Data Current Yes
Entity Filer Category Accelerated Filer
Entity Shell Company false
Entity Common Stock, Shares Outstanding 100
Central Index Key 0002115119
Current Fiscal Year End Date --12-31
Document Period End Date Mar. 31, 2026
Document Fiscal Year Focus 2026
Document Fiscal Period Focus Q1
Amendment Flag false
Entity Small Business false
Entity Emerging Growth Company false