Accrued Payroll and Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Employee deferred compensation plans	$ 421,198	$ 333,528
Payroll and benefits	280,918	263,072
Payroll taxes	21,831	23,918
Workers’ compensation	19,655	18,251
Accrued payroll and benefit costs	743,602	638,769
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Asset value of nonqualified plans	398,442	311,708
Employee deferred compensation plans	421,198	333,528
Net unrealized gains (losses) on nonqualified plan assets and liabilities	44,200	(26,600)	$ 19,400
Contribution expense for qualified and nonqualified plans	26,100	24,200	$ 21,100
Nonqualified Plan
Payables and Accruals [Abstract]
Employee deferred compensation plans	421,200	333,500
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Asset value of nonqualified plans	398,400	311,700
Employee deferred compensation plans	421,200	333,500
Nonqualified Plan | Chief Executive Officer
Payables and Accruals [Abstract]
Employee deferred compensation plans	91,800	89,200
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employee deferred compensation plans	$ 91,800	$ 89,200