| Document and Entity Information | 12 Months Ended |
| Dec. 31, 2023 | |
| Cover [Abstract] | |
| Document Type | PRE 14A |
| Amendment Flag | false |
| Entity Registrant Name | Barrett Business Services, Inc. |
| Entity Central Index Key | 0000902791 |
| Pay vs Performance Disclosure - USD ($) | 2 Months Ended | 10 Months Ended | 12 Months Ended | |||
| Mar. 05, 2020 | Dec. 31, 2020 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | |
| Pay vs Performance Disclosure | ||||||
| Pay vs Performance Disclosure, Table | Pay Versus Performance Summary Compensation Table Total for the Average Summary Compensation Table Total for Average Compensation Value of Initial Fixed $100 Investment based on: Year Principal (1) Compensation Actually (1) Summary Compensation Table Total for the PEO ⁽²⁾ Compensation Actually Paid to the PEO ⁽²⁾ Non-PEO Named Executive Officers ("NEO's") (3) Actually Paid to Non-PEO NEOs (3) Total Shareholder Return Peer Group Total Shareholder Return (4) Net Income (in thousands) (5) Gross Billings (in thousands) (6) 2023 $ 3,971,805 $ 6,334,933 N/A N/A $ 1,457,347 $ 2,088,313 $ 137 $ 121 $ 50,612 $ 7,716,152 2022 $ 4,053,415 $ 6,968,326 N/A N/A $ 1,543,168 $ 2,418,831 $ 109 $ 114 $ 47,268 $ 7,393,808 2021 $ 3,014,992 $ 4,080,497 N/A N/A $ 1,365,904 $ 1,639,683 $ 79 $ 152 $ 38,079 $ 6,569,986 2020 $ 1,951,055 $ 512,462 $ 1,353,477 $ ( 2,489,129 ) $ 969,239 $ ( 63,772 ) $ 77 $ 101 $ 33,765 $ 5,924,539 (1) Except as otherwise provided above, Mr. Kramer was the PEO for all four years presented in the table. Mr. Elich was the PEO until he retired from the Company on March 5, 2020. A reconciliation of the Summary Compensation Table and Compensation Actually Paid totals for 2023 is as follows: Current PEO SCT Total to CAP Reconciliation 2023 Total Compensation as reported in Summary Compensation Table $ 3,971,805 Subtract grant date fair value of equity awards granted during 2023 $ ( 1,926,165 ) Add year-end fair value of equity awards granted in 2023 that are outstanding and unvested as of last day of 2023 $ 2,536,136 Add/subtract change in fair value from last day of prior year to last day of 2023 of any awards granted in prior years that are outstanding and unvested as of last day of 2023 $ 1,769,451 Add/subtract the amount of change in fair value from last day of prior year to the vesting date for awards granted in prior years that vested in 2023. $ ( 13,962 ) Subtract the amount of fair value at the end of the prior fiscal year for awards granted in prior years that forfeited during 2023. $ ( 2,332 ) Compensation Actually Paid to PEO $ 6,334,933 (2) Mr. Elich was the PEO until he retired from the Company on March 5, 2020. (3) For 2021, 2022, and 2023, Mr. Harris, Mr. Blotz, and Mr. Potts were the Non-PEO NEOs. For 2020, Mr. Harris, Mr. Blotz, Mr. Potts, and Heather Gould were the Non-PEO NEOs. A reconciliation of the Summary Compensation Table and Compensation Actually Paid totals for 2023 is as follows: NEO Average SCT Total to CAP Reconciliation 2023 Total Compensation as reported in Summary Compensation Table $ 1,457,347 Subtract grant date fair value of equity awards granted during 2023 $ ( 521,703 ) Add year-end fair value of equity awards granted in 2023 that are outstanding and unvested as of last day of 2023 $ 688,392 Add/subtract change in fair value from last day of prior year to last day of 2023 of any awards granted in prior years that are outstanding and unvested as of last day of 2023 $ 474,872 Add/subtract the amount of change in fair value from last day of prior year to the vesting date for awards granted in prior years that vested in 2023. $ ( 9,973 ) Subtract the amount of fair value at the end of the prior fiscal year for awards granted in prior years that forfeited during 2023. $ ( 622 ) Compensation Actually Paid to non-PEO NEOs $ 2,088,313 (4) Peer Group Total Shareholder Return is calculated based on the S&P 1500 Human Resource & Employment Services Index, a published industry index that is considered reflective of the Company's peers, for each respective year shown in the table, assuming an initial investment of $100 on December 31, 2019. (5) Reflects after-tax net income prepared in accordance with GAAP for each of the years shown. (6) Gross billings is the financial measure in the Company’s assessment that represents the most important performance measure used to link Company performance to compensation actually paid to our PEOs and other NEOs, as it is the primary metric used in understanding the volume of the Company's business and serves as an important performance metric in managing the Company's operations, including the preparation of internal operating forecasts and establishing executive compensation performance goals. We calculate gross billings by including direct payroll costs that would otherwise be excluded from professional employer organization revenue because we are not the primary obligor for wage payments to our clients' employees. | |||||
| Company Selected Measure Name | Gross billings | |||||
| Named Executive Officers, Footnote | (1) Except as otherwise provided above, Mr. Kramer was the PEO for all four years presented in the table. Mr. Elich was the PEO until he retired from the Company on March 5, 2020. | |||||
| Peer Group Issuers, Footnote | (4) Peer Group Total Shareholder Return is calculated based on the S&P 1500 Human Resource & Employment Services Index, a published industry index that is considered reflective of the Company's peers, for each respective year shown in the table, assuming an initial investment of $100 on December 31, 2019. | |||||
| Adjustment To PEO Compensation, Footnote | (1) Except as otherwise provided above, Mr. Kramer was the PEO for all four years presented in the table. Mr. Elich was the PEO until he retired from the Company on March 5, 2020. A reconciliation of the Summary Compensation Table and Compensation Actually Paid totals for 2023 is as follows: Current PEO SCT Total to CAP Reconciliation 2023 Total Compensation as reported in Summary Compensation Table $ 3,971,805 Subtract grant date fair value of equity awards granted during 2023 $ ( 1,926,165 ) Add year-end fair value of equity awards granted in 2023 that are outstanding and unvested as of last day of 2023 $ 2,536,136 Add/subtract change in fair value from last day of prior year to last day of 2023 of any awards granted in prior years that are outstanding and unvested as of last day of 2023 $ 1,769,451 Add/subtract the amount of change in fair value from last day of prior year to the vesting date for awards granted in prior years that vested in 2023. $ ( 13,962 ) Subtract the amount of fair value at the end of the prior fiscal year for awards granted in prior years that forfeited during 2023. $ ( 2,332 ) Compensation Actually Paid to PEO $ 6,334,933 (2) Mr. Elich was the PEO until he retired from the Company on March 5, 2020. | |||||
| Adjustment to Non-PEO NEO Compensation Footnote | (3) For 2021, 2022, and 2023, Mr. Harris, Mr. Blotz, and Mr. Potts were the Non-PEO NEOs. For 2020, Mr. Harris, Mr. Blotz, Mr. Potts, and Heather Gould were the Non-PEO NEOs. A reconciliation of the Summary Compensation Table and Compensation Actually Paid totals for 2023 is as follows: NEO Average SCT Total to CAP Reconciliation 2023 Total Compensation as reported in Summary Compensation Table $ 1,457,347 Subtract grant date fair value of equity awards granted during 2023 $ ( 521,703 ) Add year-end fair value of equity awards granted in 2023 that are outstanding and unvested as of last day of 2023 $ 688,392 Add/subtract change in fair value from last day of prior year to last day of 2023 of any awards granted in prior years that are outstanding and unvested as of last day of 2023 $ 474,872 Add/subtract the amount of change in fair value from last day of prior year to the vesting date for awards granted in prior years that vested in 2023. $ ( 9,973 ) Subtract the amount of fair value at the end of the prior fiscal year for awards granted in prior years that forfeited during 2023. $ ( 622 ) Compensation Actually Paid to non-PEO NEOs $ 2,088,313 | |||||
| Compensation Actually Paid vs. Company Selected Measure | Relationship between Compensation Actually Paid and Financial Performance Measures Compensation Actually Paid ("CAP") to Mr. Kramer and total shareholder return were both higher in 2023, 2022 and 2021 compared to 2020. The increase in Mr. Kramer’s compensation is primarily the result of the increase in the Company’s stock price between 2020 and 2023. The average NEO CAP was also higher in 2023, 2022 and 2021 compared to 2020, primarily due to the increase in the Company’s stock price during this time. Total Shareholder Return increased from 2020 to 2023 primarily due to the increase in the Company’s stock price during this period. Net Income and Gross Billings also increased between 2020 and 2023. Each of these metrics is directionally aligned with the increases in CAP for both Mr. Kramer and the other NEOs during the same period. While the Company's Total Shareholder Return increased each year from 2020 to 2023, Total Shareholder Return for the Company's peer group increased between 2020 and 2021 as well as 2022 and 2023, but decreased from 2021 to 2022. | |||||
| Tabular List, Table | Most Important Measures (Unranked) Gross Billings Net Income Before Taxes Gross Margin as a percentage of Gross Billings Net Income Growth in Gross Billings | |||||
| Total Shareholder Return Amount | $ 137 | $ 109 | $ 79 | $ 77 | ||
| Peer Group Total Shareholder Return Amount | 121 | 114 | 152 | 101 | ||
| Net Income (Loss) | $ 50,612,000 | $ 47,268,000 | $ 38,079,000 | $ 33,765,000 | ||
| Company Selected Measure Amount | 7,716,152,000 | 7,393,808,000 | 6,569,986,000 | 5,924,539,000 | ||
| PEO Name | Mr. Elich | Mr. Kramer | Mr. Kramer | Mr. Kramer | Mr. Kramer | |
| Measure:: 1 | ||||||
| Pay vs Performance Disclosure | ||||||
| Name | Gross Billings | |||||
| Non-GAAP Measure Description | (6) Gross billings is the financial measure in the Company’s assessment that represents the most important performance measure used to link Company performance to compensation actually paid to our PEOs and other NEOs, as it is the primary metric used in understanding the volume of the Company's business and serves as an important performance metric in managing the Company's operations, including the preparation of internal operating forecasts and establishing executive compensation performance goals. We calculate gross billings by including direct payroll costs that would otherwise be excluded from professional employer organization revenue because we are not the primary obligor for wage payments to our clients' employees. | |||||
| Measure:: 2 | ||||||
| Pay vs Performance Disclosure | ||||||
| Name | Net Income Before Taxes | |||||
| Measure:: 3 | ||||||
| Pay vs Performance Disclosure | ||||||
| Name | Gross Margin as a percentage of Gross Billings | |||||
| Measure:: 4 | ||||||
| Pay vs Performance Disclosure | ||||||
| Name | Net Income | |||||
| Measure:: 5 | ||||||
| Pay vs Performance Disclosure | ||||||
| Name | Growth in Gross Billings | |||||
| Mr. Kramer [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| PEO Total Compensation Amount | $ 3,971,805 | $ 4,053,415 | $ 3,014,992 | $ 1,951,055 | ||
| PEO Actually Paid Compensation Amount | 6,334,933 | 6,968,326 | 4,080,497 | 512,462 | ||
| Mr. Elich [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| PEO Total Compensation Amount | $ 1,353,477 | |||||
| PEO Actually Paid Compensation Amount | $ (2,489,129) | |||||
| Mr. Harris, Mr. Blotz and Mr. Potts [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Non-PEO NEO Average Total Compensation Amount | 1,457,347 | 1,543,168 | 1,365,904 | |||
| Non-PEO NEO Average Compensation Actually Paid Amount | 2,088,313 | $ 2,418,831 | $ 1,639,683 | |||
| Mr. Harris, Mr. Blotz, Mr. Potts, and Heather Gould [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Non-PEO NEO Average Total Compensation Amount | 969,239 | |||||
| Non-PEO NEO Average Compensation Actually Paid Amount | $ (63,772) | |||||
| PEO | Mr. Kramer [Member] | Amount for Date of Grant Fair Value of Stock Awards [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | (1,926,165) | |||||
| PEO | Mr. Kramer [Member] | Fair Value of Stock Awards Granted In Year and Remain Outstanding at End of Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | 2,536,136 | |||||
| PEO | Mr. Kramer [Member] | Change in Fair Value of Awards Granted in Prior Years That Are Outstanding and Unvested at End of Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | 1,769,451 | |||||
| PEO | Mr. Kramer [Member] | Change in Fair Value of Stock Awards Granted in Prior Years and Vested During Current Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | (13,962) | |||||
| PEO | Mr. Kramer [Member] | Amount of Fair Value at the End of the Prior Fiscal Year for Awards Granted in Prior Years That Forfeited During Current Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | (2,332) | |||||
| Non-PEO NEO | Mr. Harris, Mr. Blotz and Mr. Potts [Member] | Amount for Date of Grant Fair Value of Stock Awards [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | (521,703) | |||||
| Non-PEO NEO | Mr. Harris, Mr. Blotz and Mr. Potts [Member] | Fair Value of Stock Awards Granted In Year and Remain Outstanding at End of Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | 688,392 | |||||
| Non-PEO NEO | Mr. Harris, Mr. Blotz and Mr. Potts [Member] | Change in Fair Value of Awards Granted in Prior Years That Are Outstanding and Unvested at End of Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | 474,872 | |||||
| Non-PEO NEO | Mr. Harris, Mr. Blotz and Mr. Potts [Member] | Change in Fair Value of Stock Awards Granted in Prior Years and Vested During Current Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | (9,973) | |||||
| Non-PEO NEO | Mr. Harris, Mr. Blotz and Mr. Potts [Member] | Amount of Fair Value at the End of the Prior Fiscal Year for Awards Granted in Prior Years That Forfeited During Current Year [Member] | ||||||
| Pay vs Performance Disclosure | ||||||
| Adjustment to Compensation, Amount | $ (622) | |||||
