| Document and Entity Information | May 21, 2020 |
| Cover [Abstract] | |
| Entity Address, State or Province | WA |
| Amendment Flag | false |
| Entity Central Index Key | 0000887343 |
| Document Type | 8-K |
| Document Period End Date | May 21, 2020 |
| Entity Registrant Name | COLUMBIA BANKING SYSTEM, INC. |
| Entity Incorporation State Country Code | WA |
| Entity File Number | 000-20288 |
| Entity Tax Identification Number | 91-1422237 |
| Entity Address, Address Line One | 1301 A Street |
| Entity Address, City or Town | Tacoma |
| Entity Address, Postal Zip Code | 98402 |
| City Area Code | (253) |
| Local Phone Number | 305-1900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, No Par Value |
| Trading Symbol | COLB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
