Document - shares	9 Months Ended
	Sep. 30, 2019	Oct. 31, 2019
Entity Information [Line Items]
Entity Emerging Growth Company	false
Entity Address, City or Town	Tacoma
Entity Address, State or Province	WA
Entity Address, Postal Zip Code	98402-2156
City Area Code	253
Security Exchange Name	NASDAQ
Entity Small Business	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Central Index Key	0000887343
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		72,136,242
Entity Address, Address Line One	1301 A Street
Document Type	10-Q
Trading Symbol	COLB
Entity Current Reporting Status	Yes
Entity Registrant Name	COLUMBIA BANKING SYSTEM, INC.
Entity Interactive Data Current	Yes
Local Phone Number	305-1900
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Entity File Number	0-20288
Entity Tax Identification Number	91-1422237
Entity Incorporation, State or Country Code	WA
Title of 12(b) Security	Common Stock, No Par Value
Document Quarterly Report	true
Document Transition Report	false