Business Segments - Revenue and Long-Lived Assets by Geographic Location (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
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues	$ 1,481,658	$ 1,466,226	$ 1,457,054	$ 1,395,333	$ 1,346,282	$ 1,324,709	$ 1,308,428	$ 1,287,370	$ 5,800,271	$ 5,266,789	$ 5,250,399
Assets, long-lived	125,176				144,887				125,176	144,887	161,509
Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									4,433,767	4,121,701	4,220,477
Assets, long-lived	96,169				113,069				96,169	113,069	136,434
Foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									1,366,504	1,145,088	1,029,922
Assets, long-lived	$ 29,007				$ 31,818				$ 29,007	$ 31,818	$ 25,075