Stockholders' Equity - Summary of Activity Related to Stock Option Plans (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares available for grant, beginning balance (in shares)	8,699,941
Options outstanding, number of shares, beginning balance (in shares)	20,479,278
Shares available for grant, granted (in shares)	619,636
Options outstanding, number of shares, exercised (in shares)	(593,294)
Options outstanding, number of shares expired (in shares)	(21)
Shares available for grant, ending balance (in shares)	8,080,305
Options outstanding, number of shares, ending balance (in shares)	20,505,599
Options vested and exercisable, number of shares, ending balance (in shares)	20,505,599
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	$ 89.61
Options outstanding, weighted-average exercise price, granted (dollars per share) | $ / shares	319.43
Options outstanding, weighted-average exercise price, exercised (dollars per share) | $ / shares	38.07
Options expired, weighted-average exercise price (dollars per share) | $ / shares	4.27
Options outstanding, weighted-average exercise price, ending balance (dollars per share) | $ / shares	98.04
Options outstanding, weighted-average exercise price, vested and exercisable (dollars per share) | $ / shares	$ 98.04
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average remaining contractual term, ending balance (in years)	5 years 8 months 5 days
Weighted-average remaining contractual term, vested and exercisable (in years)	5 years 8 months 5 days
Aggregate intrinsic value, ending balance | $	$ 5,312,077
Aggregate intrinsic value, vested and exercisable | $	$ 5,312,077