Quarterly Financial Results (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]
Revenue	$ 846,287	$ 792,845	$ 794,715	$ 764,302	$ 772,123	$ 709,912	$ 705,074	$ 706,508	$ 3,198,149	$ 2,893,617	$ 2,714,474
Cost of revenue (exclusive of amortization of acquired intangible assets shown below)	303,847	283,439	276,804	268,582	257,750	246,938	242,193	240,743	1,132,672	987,624	953,485
Net income	$ 113,370	$ 158,623	$ 161,915	$ 123,146	$ 119,100	$ 137,890	$ 113,915	$ 107,130	$ 557,054	$ 478,035	$ 298,373
Basic net income per share (in dollars per share)	$ 0.70	$ 0.97	$ 1.00	$ 0.76	$ 0.74	$ 0.85	$ 0.70	$ 0.66	$ 3.43	$ 2.94	$ 1.78
Diluted net income per share (in dollars per share)	$ 0.68	$ 0.95	$ 0.98	$ 0.75	$ 0.73	$ 0.84	$ 0.69	$ 0.65	$ 3.37	$ 2.90	$ 1.76