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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 94,015	$ 107,583	$ 43,061	$ 53,714	$ 27,520	$ 63,911	$ 56,752	$ 74,583	$ 298,373	$ 222,766	$ 320,727
Denominator:
Shares used for basic net income per share (in shares)									167,312	171,559	174,917
Effect of dilutive securities:
Stock options (in shares)									132	260	384
RSUs and deferred stock units (in shares)									1,744	892	914
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)									169,188	172,711	176,215
Basic net income per share (in dollars per share)	$ 0.58	$ 0.65	$ 0.25	$ 0.32	$ 0.16	$ 0.37	$ 0.33	$ 0.43	$ 1.78	$ 1.30	$ 1.83
Diluted net income per share (in dollars per share)	$ 0.57	$ 0.64	$ 0.25	$ 0.31	$ 0.16	$ 0.37	$ 0.33	$ 0.43	$ 1.76	$ 1.29	$ 1.82