Document and Entity Information	6 Months Ended
	Mar. 31, 2018 shares
Document and Entity Information [Abstract]
Entity Registrant Name	Columbia Financial, Inc.
Entity Central Index Key	0001723596
Current Fiscal Year End Date	--09-30
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period	Q2
Trading Symbol	CLBK
Amendment Flag	false
Entity Common Stock, Shares Outstanding	10