Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 22, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Period End Date	Dec. 31, 2021
Document Annual Report	true
Current Fiscal Year End Date	--12-31
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, par value $0.01 per share
Trading Symbol	CLBK
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Filer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 590.4
Entity Common Stock, Shares Outstanding (in shares)		106,701,453
Documents Incorporated by Reference	NONE
Entity Central Index Key	0001723596
Document Fiscal Year Focus	2021
Document Fiscal Period	FY
Amendment Flag	false