Accrued Payroll and Benefit Costs - Accrued Payroll/Benefit Costs and Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Payroll and benefits	$ 477,121	$ 311,169
Payroll taxes	60,799	67,712
Workers’ compensation	19,993	18,996
Accrued payroll and benefit costs	557,913	397,877
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Applicable payroll taxes deferred	102,200	$ 102,200
Accrued Payroll and Benefits Costs
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Applicable payroll taxes deferred	51,100
Other Liabilities
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Applicable payroll taxes deferred	$ 51,100