Business Segments - Revenue and Long-Lived Assets by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues	$ 1,346,282	$ 1,324,709	$ 1,308,428	$ 1,287,370	$ 1,265,073	$ 1,338,541	$ 1,344,160	$ 1,302,625	$ 5,266,789	$ 5,250,399	$ 5,094,933
Assets, long-lived	144,887				161,509				144,887	161,509	142,906
Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									4,121,701	4,220,477	4,105,013
Assets, long-lived	113,069				136,434				113,069	136,434	117,176
Foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Net service revenues									1,145,088	1,029,922	989,920
Assets, long-lived	$ 31,818				$ 25,075				$ 31,818	$ 25,075	$ 25,730