Quarterly Financial Results (unaudited) - Schedule of Quarterly Financial Results (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 536,295	$ 498,042	$ 476,035	$ 453,502	$ 435,980	$ 395,790	$ 378,106	$ 368,046	$ 1,963,874	$ 1,577,922	$ 1,373,947
Cost of revenue (exclusive of amortization of acquired intangible assets shown below)	163,201	158,812	149,318	139,612	133,951	132,039	124,705	120,392	610,943	511,087	529,900
Net income	$ 97,107	$ 91,155	$ 72,886	$ 72,800	$ 80,349	$ 79,756	$ 61,895	$ 71,487	$ 333,948	$ 293,487	$ 203,989
Basic net income per share (in dollars per share)	$ 0.55	$ 0.51	$ 0.41	$ 0.41	$ 0.45	$ 0.45	$ 0.35	$ 0.40	$ 1.87	$ 1.65	$ 1.15
Diluted net income per share (in dollars per share)	$ 0.54	$ 0.50	$ 0.40	$ 0.40	$ 0.44	$ 0.44	$ 0.34	$ 0.39	$ 1.84	$ 1.61	$ 1.12