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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income (in dollars)	$ 107,583	$ 63,911	$ 204,358	$ 195,246
Denominator:
Shares used for basic net income per share (in shares)	165,924	170,976	168,763	172,269
Effect of dilutive securities:
Stock options (in shares)	80	150	158	279
RSUs and DSUs (in shares)	1,896	379	1,811	823
Convertible senior notes (in shares)	0	0	0	0
Warrants related to issuance of convertible senior notes (in shares)	0	0	0	0
Shares used for diluted net income per share (in shares)	167,900	171,505	170,732	173,371
Basic net income per share (in dollars per share)	$ 0.65	$ 0.37	$ 1.21	$ 1.13
Diluted net income per share (in dollars per share)	$ 0.64	$ 0.37	$ 1.20	$ 1.13