Net Income Per Share Available to Common Stock - Basic and Diluted Net Income Per 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
Calculation of Basic and Diluted Net Income Per Share, Numerator [Abstract]
Net income	$ 170,322	$ 168,524	$ (155,269)	$ 144,538	$ 103,833	$ 133,722	$ 102,685	$ 82,961	$ 328,115	$ 423,201	$ 184,189
Less: undistributed earnings allocated to participating securities									(189)	(801)	(1,224)
Net income attributable to common stockholders, basic									327,926	422,400	182,965
Add: undistributed earnings allocated to participating securities									15	68	74
Net income attributable to common stockholders, diluted									$ 327,941	$ 422,468	$ 183,039
Calculation of Basic and Diluted Net Income Per Share, Denominator [Abstract]
Weighted-average shares used in computing net income per share available to common stockholders, basic (in shares)									74,750	72,258	68,771
Add weighted-average effect of dilutive securities:
Stock options, RSUs and RSAs (in shares)									6,083	6,599	4,408
Employee stock purchase plan (in shares)									11	120	43
Weighted-average shares used in computing net income per share available to common stockholders, diluted (in shares)									80,844	78,977	73,222
Net income per share attributable to common stockholders:
Basic (in dollars per share)	$ 2.26	$ 2.25	$ (2.08)	$ 1.95	$ 1.42	$ 1.84	$ 1.42	$ 1.16	$ 4.39	$ 5.85	$ 2.66
Diluted (in dollars per share)	$ 2.10	$ 2.08	$ (2.08)	$ 1.79	$ 1.29	$ 1.68	$ 1.30	$ 1.07	$ 4.06	$ 5.35	$ 2.50