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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Calculation of Basic and Diluted Net Income Per Share, Numerator [Abstract]
Net income	$ 168,524	$ 133,722	$ 157,793	$ 319,368
Less: undistributed earnings allocated to participating securities	(85)	(182)	(87)	(725)
Net income attributable to common stockholders, basic	168,439	133,540	157,706	318,643
Net income attributable to common stockholders, basic	168,439	133,540	157,706	318,643
Add: undistributed earnings allocated to participating securities	6	15	7	61
Net income attributable to common stockholders, diluted	$ 168,445	$ 133,555	$ 157,713	$ 318,704
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)	75,011	72,588	74,506	71,903
Add weighted-average effect of dilutive securities:
Stock options and RSUs (in shares)	5,967	6,636	6,298	6,528
Employee stock purchase plan (in shares)	40	98	40	97
Weighted-average shares used in computing net income per share available to common stockholders, diluted (in shares)	81,018	79,322	80,844	78,528
Net income per share attributable to common stockholders:
Basic (in dollars per share)	$ 2.25	$ 1.84	$ 2.12	$ 4.43
Diluted (in dollars per share)	$ 2.08	$ 1.68	$ 1.95	$ 4.06