Quarterly Financial Results (unaudited) - Schedule of Quarterly Financial Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 663,452	$ 621,399	$ 608,908	$ 609,237	$ 616,124	$ 584,065	$ 572,135	$ 567,725	$ 2,502,996	$ 2,340,049	$ 2,197,448
Cost of revenue (exclusive of amortization of acquired intangible assets)	229,937	225,468	214,650	205,703	203,475	204,467	206,323	194,736	875,758	809,001	725,620
Net income	$ 19,107	$ 60,512	$ 57,772	$ 80,930	$ 91,639	$ 76,000	$ 73,635	$ 74,858	$ 218,321	$ 316,132	$ 321,406
Basic net income per share (in dollars per share)	$ 0.11	$ 0.35	$ 0.33	$ 0.47	$ 0.53	$ 0.44	$ 0.42	$ 0.42	$ 1.27	$ 1.81	$ 1.80
Diluted net income per share (in dollars per share)	$ 0.11	$ 0.35	$ 0.33	$ 0.46	$ 0.52	$ 0.43	$ 0.42	$ 0.42	$ 1.26	$ 1.79	$ 1.78