| Cover | Nov. 03, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 3, 2020 |
| Entity File Number | 0-21886 |
| Entity Registrant Name | BARRETT BUSINESS SERVICES, INC. |
| Entity Central Index Key | 0000902791 |
| Entity Tax Identification Number | 52-0812977 |
| Entity Incorporation, State or Country Code | MD |
| Entity Address, Address Line One | 8100 N.E. Parkway Drive |
| Entity Address, Address Line Two | Suite 200 |
| Entity Address, City or Town | Vancouver |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98662 |
| City Area Code | 360 |
| Local Phone Number | 828-0700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | BBSI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
