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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income (in dollars)	$ 43,061	$ 56,752	$ 96,775	$ 131,335
Denominator:
Shares used for basic net income per share (in shares)	170,250	172,674	170,183	172,916
Effect of dilutive securities:
Stock options (in shares)	268	329	205	344
RSUs and DSUs (in shares)	1,789	436	1,768	1,045
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)	172,307	173,439	172,156	174,305
Basic net income per share (in dollars per share)	$ 0.25	$ 0.33	$ 0.57	$ 0.76
Diluted net income per share (in dollars per share)	$ 0.25	$ 0.33	$ 0.56	$ 0.75