Accrued Payroll and Benefit Costs (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Payroll and benefits	$ 256,804	$ 243,301
Employee deferred compensation plans	312,429	252,349
Workers’ compensation	17,092	19,361
Payroll taxes	26,574	24,037
Accrued payroll and benefit costs	612,899	539,048
Chief Executive Officer
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred compensation plan and other benefits related to the Company’s Chief Executive Officer	$ 86,145	$ 83,899