Stockholders' Equity - Summary of Activity Related to Stock Option Plans (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 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	1,507,700
Options outstanding, number of shares, exercised	(2,692,309)
Options outstanding, number of shares expired	(2,051)
Shares available for grant, ending balance	9,232,215
Options outstanding, number of shares, ending balance	20,460,690
Options vested and exercisable, number of shares, ending balance	20,460,690
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	306.98
Options outstanding, weighted-average exercise price, exercised (dollars per share) | $ / shares	41.75
Options expired, weighted-average exercise price (dollars per share) | $ / shares	4.64
Options outstanding, weighted-average exercise price, ending balance (dollars per share) | $ / shares	81.80
Options outstanding, weighted-average exercise price, vested and exercisable (dollars per share) | $ / shares	$ 81.80
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average remaining contractual term, ending balance (in years)	5 years 9 months 4 days
Weighted-average remaining contractual term, vested and exercisable (in years)	5 years 9 months 4 days
Aggregate intrinsic value, ending balance | $	$ 5,984,520
Aggregate intrinsic value, vested and exercisable | $	$ 5,984,520