Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 09, 2019
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 Ex Transition Period	false
Entity Small Business	false
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period	Q1
Trading Symbol	CLBK
Amendment Flag	false
Entity Common Stock, Shares Outstanding (in shares)		115,889,175