v3.26.1
Cover Page - shares
3 Months Ended
Mar. 31, 2026
May 07, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Transition Report false  
Entity File Number 001-38456  
Entity Registrant Name Columbia Financial, Inc.  
Entity Incorporation, State or Country Code DE  
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  
Title of 12(b) Security Common Stock, $0.01 par value per share  
Trading Symbol CLBK  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding (in shares)   104,047,469
Entity Central Index Key 0001723596  
Current Fiscal Year End Date --12-31  
Document Period End Date Mar. 31, 2026  
Document Fiscal Year Focus 2026  
Document Fiscal Period Q1  
Amendment Flag false