Description of Business and Significant Accounting Policies - Correction of Immaterial Errors (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues	$ 446,960	$ 455,606	$ 462,101	$ 476,111	$ 494,952	$ 505,500	$ 501,587	$ 500,562	$ 1,840,778	$ 2,002,601	$ 1,861,206
Costs of services provided									1,612,877	1,768,162	1,610,590
(Loss) income before income taxes	$ 25,871	$ 23,716	$ 23,617	$ 11,892	$ 35,007	$ 32,982	$ 33,316	$ (1,395)	$ 85,096	99,910	132,965
Adjustment for captive insurance company
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues										6,200	4,900
Costs of services provided										3,800	1,900
(Loss) income before income taxes										$ 2,400	$ 3,000