Net Income per Share - Schedule of Components Used in Diluted and Basic Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income	$ 91,639	$ 76,000	$ 73,635	$ 74,858	$ 88,420	$ 88,040	$ 67,200	$ 77,746	$ 316,132	$ 321,406	$ 333,948
Denominator:
Shares used for basic net income per share (in shares)									174,917	178,391	178,279
Effect of dilutive securities:
Stock options (in shares)									384	794	1,221
RSUs and deferred stock units (in shares)									914	1,230	1,686
Convertible senior notes (in shares)									0	0	0
Warrants related to issuance of convertible senior notes (in shares)									0	0	0
Shares used for diluted net income per share (in shares)									176,215	180,415	181,186
Basic net income per share (in dollars per share)	$ 0.53	$ 0.44	$ 0.42	$ 0.42	$ 0.50	$ 0.49	$ 0.38	$ 0.44	$ 1.81	$ 1.80	$ 1.87
Diluted net income per share (in dollars per share)	$ 0.52	$ 0.43	$ 0.42	$ 0.42	$ 0.49	$ 0.49	$ 0.37	$ 0.43	$ 1.79	$ 1.78	$ 1.84