Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
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
Gross margin	638,944	646,446	637,541	607,588	620,062	610,468	607,118	572,366	2,530,519	2,410,014	2,163,812
Income before income taxes	154,247	163,782	160,103	147,383	154,983	151,905	150,075	134,639	625,515	591,602	517,516
Net income	$ 112,842	$ 117,181	$ 114,612	$ 109,798	$ 113,564	$ 115,242	$ 109,315	$ 96,167	$ 454,433	$ 434,288	$ 290,584
Basic net income per share (in USD per share)	$ 0.99	$ 1.02	$ 0.98	$ 0.94	$ 0.96	$ 0.96	$ 0.90	$ 0.79	$ 3.93	$ 3.60	$ 2.34
Diluted net income per share (in USD per share)	$ 0.98	$ 1.01	$ 0.98	$ 0.93	$ 0.95	$ 0.95	$ 0.89	$ 0.78	$ 3.90	$ 3.57	$ 2.33