Summary of Significant Accounting Policies - Concentration of Business (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating results by business segment
Net revenue	$ 1,844,645	$ 1,853,506	$ 1,810,923	$ 1,805,077	$ 1,694,415	$ 1,419,406	$ 1,419,139	$ 1,305,623	$ 7,314,151	$ 5,838,583	$ 4,836,884
Minimum
Operating results by business segment
Term of contracts with customers									1 year
Term of renewal contracts									1 year
Notice period for termination of contract									30 days
Maximum
Operating results by business segment
Term of contracts with customers									3 years
Term of renewal contracts									2 years
Notice period for termination of contract									180 days
Pennsylvania Counties
Operating results by business segment
Net revenue									$ 544,600	490,000	461,600
CMS
Operating results by business segment
Net revenue									442,300	511,000	272,800
United States federal government
Operating results by business segment
Net revenue									$ 308,700	$ 341,500	$ 252,500