Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 29, 2019	Jun. 29, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Columbia Financial, Inc.
Entity Central Index Key	0001723596
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Small Business	false
Entity Shell Company	false
Entity Ex Transition Period	false
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period	FY
Trading Symbol	CLBK
Amendment Flag	false
Entity Common Stock, Shares Outstanding (in shares)		115,889,175
Entity Well-known Seasoned Filer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 738.2