Business Segments - Revenue and Long-Lived Assets by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Service revenues	$ 1,537,385	$ 1,552,132	$ 1,516,385	$ 1,468,530	$ 1,481,658	$ 1,466,226	$ 1,457,054	$ 1,395,333	$ 6,074,432	$ 5,800,271	$ 5,266,789
Assets, long-lived	128,385				125,176				128,385	125,176	144,887
Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Service revenues									4,708,715	4,433,767	4,121,701
Assets, long-lived	99,365				96,169				99,365	96,169	113,069
Foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Service revenues									1,365,717	1,366,504	1,145,088
Assets, long-lived	$ 29,020				$ 29,007				$ 29,020	$ 29,007	$ 31,818