Business Segments - Revenue and Long-Lived Assets by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues	$ 1,304,594	$ 1,312,718	$ 1,272,058	$ 1,205,563	$ 1,221,450	$ 1,224,308	$ 1,164,914	$ 1,084,342	$ 5,094,933	$ 4,695,014	$ 4,245,895
Assets, long-lived	142,906				121,754				142,906	121,754	112,644
Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									4,105,013	3,623,812	3,219,820
Assets, long-lived	117,176				101,181				117,176	101,181	92,252
Foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									989,920	1,071,202	1,026,075
Assets, long-lived	$ 25,730				$ 20,573				$ 25,730	$ 20,573	$ 20,392