Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 30, 2019
Document Information [Line Items]
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Entity Registrant Name	COLUMBIA BANKING SYSTEM INC
Trading Symbol	COLB
Amendment Flag	false
Entity Emerging Growth Company	false
Entity Small Business	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Central Index Key	0000887343
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		73,555,751