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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income	$ 88,420	$ 88,040	$ 67,200	$ 77,746	$ 97,107	$ 91,155	$ 72,886	$ 72,800	$ 321,406	$ 333,948	$ 293,487
Denominator:
Shares used for basic net income per share (in shares)									178,391	178,279	178,196
Effect of dilutive securities:
Stock options (in shares)									794	1,221	1,622
RSUs and deferred stock units (in shares)									1,230	1,686	1,965
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)									180,415	181,186	181,783
Basic net income per share (in dollars per share)	$ 0.50	$ 0.49	$ 0.38	$ 0.44	$ 0.55	$ 0.51	$ 0.41	$ 0.41	$ 1.80	$ 1.87	$ 1.65
Diluted net income per share (in dollars per share)	$ 0.49	$ 0.49	$ 0.37	$ 0.43	$ 0.54	$ 0.50	$ 0.40	$ 0.40	$ 1.78	$ 1.84	$ 1.61