Stockholders' Equity - Summary of Activity Related to Stock Option Plans (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares available for grant, beginning balance	10,739,915
Options outstanding, number of shares, beginning balance	21,647,350
Shares available for grant, granted	(1,056,538)
Options outstanding, number of shares, exercised	(2,064,819)
Options expired, number of shares	(1,820)
Shares available for grant, ending balance	9,683,377
Options outstanding, number of shares, ending balance	20,637,249
Options vested and exercisable, number of shares, ending balance	20,637,249
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Options outstanding, weighted-average exercise price, beginning balance (dollars per share) | $ / shares	$ 61.13
Options outstanding, weighted-average exercise price, granted (dollars per share) | $ / shares	282.22
Options outstanding, weighted-average exercise price, exercised (dollars per share) | $ / shares	39.92
Options expired, weighted-average exercise price (dollars per share) | $ / shares	4.72
Options outstanding, weighted-average exercise price, ending balance (dollars per share) | $ / shares	74.58
Options outstanding, weighted-average exercise price, vested and exercisable (dollars per share) | $ / shares	$ 74.58
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average remaining contractual term, ending balance (in years)	5 years 10 months 6 days
Weighted-average remaining contractual term, vested and exercisable (in years)	5 years 10 months 6 days
Aggregate intrinsic value, ending balance | $	$ 6,538,997
Aggregate intrinsic value, vested and exercisable | $	$ 6,538,997