Business Segments - Revenue and Long-Lived Assets by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Service revenues	$ 1,304,086	$ 1,189,897	$ 1,108,326	$ 1,506,691	$ 1,537,385	$ 1,552,132	$ 1,516,385	$ 1,468,530	$ 5,109,000	$ 6,074,432	$ 5,800,271
Property and equipment, net	109,817				128,385				109,817	128,385	125,176
Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Service revenues									3,984,742	4,708,715	4,433,767
Property and equipment, net	88,117				99,365				88,117	99,365	96,169
Foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Service revenues									1,124,258	1,365,717	1,366,504
Property and equipment, net	$ 21,700				$ 29,020				$ 21,700	$ 29,020	$ 29,007