Accrued Payroll and Benefit Costs - Accrued Payroll/Benefit Costs and Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Employee deferred compensation plans	$ 465,299	$ 421,198
Payroll and benefits	314,299	280,918
Payroll taxes	26,273	21,831
Workers’ compensation	21,877	19,655
Accrued payroll and benefit costs	827,748	743,602
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred applicable payroll taxes	68,300
Asset value of nonqualified plans	445,739	398,442
Employee deferred compensation plans	465,299	421,198
Nonqualified Plan
Payables and Accruals [Abstract]
Employee deferred compensation plans	465,300	421,200
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Asset value of nonqualified plans	445,700	398,400
Employee deferred compensation plans	465,300	421,200
Nonqualified Plan | Chief Executive Officer
Payables and Accruals [Abstract]
Employee deferred compensation plans	88,600	91,800
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employee deferred compensation plans	$ 88,600	$ 91,800