Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
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
Gross margin	514,342	467,346	416,535	614,388	636,861	646,278	636,589	605,401	2,012,611	2,525,129	2,411,012
Income before income taxes	129,585	102,510	58,024	131,763	154,247	163,782	160,103	147,383	421,882	625,515	591,602
Net income	$ 94,416	$ 75,749	$ 46,196	$ 89,915	$ 112,842	$ 117,181	$ 114,612	$ 109,798	$ 306,276	$ 454,433	$ 434,288
Basic net income per share (usd per share)	$ 0.84	$ 0.67	$ 0.41	$ 0.79	$ 0.99	$ 1.02	$ 0.98	$ 0.94	$ 2.72	$ 3.93	$ 3.60
Diluted net income per share (usd per share)	$ 0.84	$ 0.67	$ 0.41	$ 0.79	$ 0.98	$ 1.01	$ 0.98	$ 0.93	$ 2.70	$ 3.90	$ 3.57