| Pay vs Performance Disclosure number in Thousands | 12 Months Ended |
| Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) |
| Pay vs Performance Disclosure | | | | |
| Pay vs Performance Disclosure, Table | Year Summary Comp Table (SCT) - PEO ($) Compensation Actually Paid (CAP) – PEO (2) ($) Average SCT Non-PEOs ($) Avg CAP - Non-PEOs (3) ($) Value of Initial Fixed $100 Investment Based on: Net Income (in thousands) ($) Revenue (in thousands) (5) ($) Total Shareholder Return ($) Peer Group Shareholder Return (4) ($) 2023 8,126,837 15,213,375 4,346,184 7,466,839 151.58 126.75 411,146 6,392,517 2022 9,391,554 (945,769) 4,226,824 (594,670) 124.11 110.84 657,919 7,238,142 2021 8,971,474 31,454,099 4,029,518 15,993,523 183.81 146.45 598,626 6,461,444 2020 6,595,507 7,439,020 3,458,542 3,620,347 101.36 98.25 306,276 5,109,000 | | | |
| Company Selected Measure Name | revenue | | | |
| Named Executive Officers, Footnote | During 2020-2023, Mr. M. Keith Waddell was our CEO. During 2020-2022, the Company’s non-PEO NEOs consisted of Mr. Michael C. Buckley, Paul F. Gentzkow, Robert W. Glass, and Harold M. Messmer, Jr. During 2023, the Company’s non-PEO NEOs consisted of Mr. Michael C. Buckley, Paul F. Gentzkow, Joseph A. Tarantino, and Harold M. Messmer, Jr. | | | |
| Peer Group Issuers, Footnote | The peer group used for calculating Peer Group Total Shareholder Return consists of the following corporations providing temporary or permanent employment services: Kelly Services, Inc.; Kforce, Inc.; ManpowerGroup; and Resources Connection Inc. This is the same group of companies used to prepare the Stock Performance Graph disclosed in the Company’s Annual Report on Form 10-K. | | | |
| PEO Total Compensation Amount | $ 8,126,837 | $ 9,391,554 | $ 8,971,474 | $ 6,595,507 |
| PEO Actually Paid Compensation Amount | $ 15,213,375 | (945,769) | 31,454,099 | 7,439,020 |
| Adjustment To PEO Compensation, Footnote | The SEC rules require that certain adjustments be made to the Summary Compensation Table totals to determine CAP, as reported in the Pay Versus Performance table above. The applicable adjustments that were made to determine CAP for the PEO are summarized in the following table: Year Summary Comp Table ($) Subtract: Stock Awards in Summary Comp Table ($) Add fair value of all awards granted during the fiscal year that are outstanding and unvested ($) Add the change in fair value of any awards granted in any prior fiscal year that are outstanding and unvested ($) Add the fair value of awards that are granted and vest in the same year ($) Add the change in fair value of any awards granted in any prior fiscal year that vested ($) Subtract the fair value of any awards granted in any prior fiscal year that fail to meet the applicable vesting conditions ($) Add any dividends or other earnings paid on awards that are not otherwise included in the total compensation for the covered fiscal year ($) Compensation Actually Paid (CAP) ($) 2023 8,126,837 (5,184,571) 9,251,809 2,764,629 — 254,671 — — 15,213,375 2022 9,391,554 (5,304,563) 4,509,735 (9,246,281) — (296,214) — — (945,769) 2021 8,971,474 (5,202,987) 12,062,350 14,239,450 — 1,383,812 — — 31,454,099 2020 6,595,507 (4,575,965) 7,360,830 458,750 — (2,400,102) — — 7,439,020 | | | |
| Non-PEO NEO Average Total Compensation Amount | $ 4,346,184 | 4,226,824 | 4,029,518 | 3,458,542 |
| Non-PEO NEO Average Compensation Actually Paid Amount | $ 7,466,839 | (594,670) | 15,993,523 | 3,620,347 |
| Adjustment to Non-PEO NEO Compensation Footnote | The SEC rules require that certain adjustments be made to the Summary Compensation Table totals to determine CAP, as reported in the Pay Versus Performance table above. The applicable adjustments that were made to determine CAP for the non-PEO NEOs is summarized in the following table: Year Summary Comp Table ($) Subtract: Stock Awards in Summary Comp Table ($) Add fair value of all awards granted during the fiscal year that are outstanding and unvested ($) Add the change in fair value of any awards granted in any prior fiscal year that are outstanding and unvested ($) Add the fair value of awards that are granted and vest in the same year ($) Add the change in fair value of any awards granted in any prior fiscal year that vested ($) Subtract the fair value of any awards granted in any prior fiscal year that fail to meet the applicable vesting conditions ($) Add any dividends or other earnings paid on awards that are not otherwise included in the total compensation for the covered fiscal year ($) Compensation Actually Paid (CAP) ($) 2023 4,346,184 (2,470,675) 4,257,733 1,175,120 — 158,477 — — 7,466,839 2022 4,226,824 (2,185,148) 1,857,728 (4,301,705) — (192,369) — — (594,670) 2021 4,029,518 (2,143,374) 4,969,092 8,239,605 — 898,682 — — 15,993,523 2020 3,458,542 (2,337,509) 3,760,082 297,925 — (1,558,693) — — 3,620,347 | | | |
| Compensation Actually Paid vs. Total Shareholder Return | | | | |
| Compensation Actually Paid vs. Net Income | | | | |
| Compensation Actually Paid vs. Company Selected Measure | | | | |
| Total Shareholder Return Vs Peer Group | | | | |
| Tabular List, Table | Most Important Financial Measures for 2023 Nature Description Revenue Financial Measure GAAP Revenue compared to target Relative ROIC Financial Measure Performance against the 65th percentile ROIC of an industry peer group over the three-year measurement period(a) Relative TSR Financial Measure Performance against the 50th percentile TSR of the industry peer group over the three-year measurement period(a) Net Income Financial Measure GAAP Net Income compared to target | | | |
| Total Shareholder Return Amount | $ 151.58 | 124.11 | 183.81 | 101.36 |
| Peer Group Total Shareholder Return Amount | 126.75 | 110.84 | 146.45 | 98.25 |
| Net Income (Loss) | $ 411,146,000 | $ 657,919,000 | $ 598,626,000 | $ 306,276,000 |
| Company Selected Measure Amount | 6,392,517 | 7,238,142 | 6,461,444 | 5,109,000 |
| PEO Name | Mr. M. Keith Waddell | | | |
| Additional 402(v) Disclosure | The Company has identified revenue as the company-selected measure for the pay versus performance disclosure, as it represents the most important performance measure not otherwise required to be disclosed in the table, which includes net income, that the Company uses to link compensation actually paid to the Company’s NEOs to Company performance for the most recently completed fiscal year. For a full description of the 2023 performance share grants, please see the 2023 Performance Design section in the CD&A. | | | |
| Measure:: 1 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Revenue | | | |
| Measure:: 2 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Relative ROIC | | | |
| Measure:: 3 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Relative TSR | | | |
| Measure:: 4 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Net Income | | | |
| PEO | Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | $ (5,184,571) | $ (5,304,563) | $ (5,202,987) | $ (4,575,965) |
| PEO | Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 9,251,809 | 4,509,735 | 12,062,350 | 7,360,830 |
| PEO | Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 2,764,629 | (9,246,281) | 14,239,450 | 458,750 |
| PEO | Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 0 | 0 | 0 | 0 |
| PEO | Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 254,671 | (296,214) | 1,383,812 | (2,400,102) |
| PEO | Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 0 | 0 | 0 | 0 |
| PEO | Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 0 | 0 | 0 | 0 |
| Non-PEO NEO | Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | (2,470,675) | (2,185,148) | (2,143,374) | (2,337,509) |
| Non-PEO NEO | Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 4,257,733 | 1,857,728 | 4,969,092 | 3,760,082 |
| Non-PEO NEO | Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 1,175,120 | (4,301,705) | 8,239,605 | 297,925 |
| Non-PEO NEO | Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 0 | 0 | 0 | 0 |
| Non-PEO NEO | Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 158,477 | (192,369) | 898,682 | (1,558,693) |
| Non-PEO NEO | Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 0 | 0 | 0 | 0 |
| Non-PEO NEO | Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | $ 0 | $ 0 | $ 0 | $ 0 |
