Accrued Payroll and Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Payables and Accruals [Abstract]
Employee deferred compensation plans	$ 428,683		$ 428,683		$ 421,198
Payroll and benefits	257,190		257,190		280,918
Payroll taxes	7,816		7,816		21,831
Workers’ compensation	21,223		21,223		19,655
Accrued payroll and benefit costs	714,912		714,912		743,602
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred applicable payroll taxes	37,500		37,500
Employee deferred compensation plans	428,683		428,683		421,198
Net unrealized gains (losses) on nonqualified plan assets and liabilities	47,800	$ 6,400	(1,000)	$ 30,100
Contribution expenses for qualified and nonqualified plans	6,700	$ 4,800	12,300	$ 8,400
Nonqualified Plan
Payables and Accruals [Abstract]
Employee deferred compensation plans	428,700		428,700		421,200
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Asset value of nonqualified plans	418,100		418,100		398,400
Employee deferred compensation plans	428,700		428,700		421,200
Nonqualified Plan | Chief Executive Officer
Payables and Accruals [Abstract]
Employee deferred compensation plans	88,300		88,300		91,800
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employee deferred compensation plans	$ 88,300		$ 88,300		$ 91,800