Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 14, 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
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period	Q3
Trading Symbol	CLBK
Amendment Flag	false
Entity Common Stock, Shares Outstanding (in shares)		115,889,175