Business Segments - Revenue and Long-Lived Assets by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues	$ 1,265,073	$ 1,338,541	$ 1,344,160	$ 1,302,625	$ 1,304,594	$ 1,312,718	$ 1,272,058	$ 1,205,563	$ 5,250,399	$ 5,094,933	$ 4,695,014
Assets, long-lived	161,509				142,906				161,509	142,906	121,754
Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									4,220,477	4,105,013	3,623,812
Assets, long-lived	136,434				117,176				136,434	117,176	101,181
Foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									1,029,922	989,920	1,071,202
Assets, long-lived	$ 25,075				$ 25,730				$ 25,075	$ 25,730	$ 20,573