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, 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
Numerator:
Net income	$ 19,107	$ 60,512	$ 57,772	$ 80,930	$ 91,639	$ 76,000	$ 73,635	$ 74,858	$ 218,321	$ 316,132	$ 321,406
Denominator:
Shares used for basic net income per share (in shares)									171,559	174,917	178,391
Effect of dilutive securities:
Stock options (in shares)									260	384	794
RSUs and deferred stock units (in shares)									892	914	1,230
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)									172,711	176,215	180,415
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