Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 26, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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
Entity Shell Company	false
Entity Public Float			$ 721.1
Entity Common Stock, Shares Outstanding (in shares)		112,361,961
Documents Incorporated by Reference	Portions of the Registrant’s Proxy Statement for the 2020 Annual Meeting of Stockholders are incorporated by reference into Part III of this Form 10-K.
Entity Central Index Key	0001723596
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period	FY
Amendment Flag	false