| Document and Entity Information | Oct. 21, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 21, 2020 |
| Entity Registrant Name | Banner Corporation |
| Entity Incorporation, State or Country Code | WA |
| Entity File Number | 000-26584 |
| Entity Tax Identification Number | 91-1691604 |
| Entity Address, Address Line One | 10 S First Avenue |
| Entity Address, City or Town | Walla Walla |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 99362 |
| City Area Code | 509 |
| Local Phone Number | 527-3636 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000946673 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | BANR |
| Security Exchange Name | NASDAQ |
