Stockholders' Equity - Activity Related to Stock Option Plans (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 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)	1,200,996
Options outstanding, number of shares, exercised (in shares)	(1,236,285)
Options outstanding, number of shares expired (in shares)	(21)
Shares available for grant, ending balance (in shares)	7,498,945
Options outstanding, number of shares, ending balance (in shares)	20,443,968
Options vested and exercisable, number of shares, ending balance (in shares)	20,443,968
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	339.01
Options outstanding, weighted-average exercise price, exercised (dollars per share) | $ / shares	36.67
Options expired, weighted-average exercise price (dollars per share) | $ / shares	4.27
Options outstanding, weighted-average exercise price, ending balance (dollars per share) | $ / shares	107.46
Options outstanding, weighted-average exercise price, vested and exercisable (dollars per share) | $ / shares	$ 107.46
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average remaining contractual term, ending balance (in years)	5 years 7 months 13 days
Weighted-average remaining contractual term, vested and exercisable (in years)	5 years 7 months 13 days
Aggregate intrinsic value, ending balance | $	$ 5,320,942
Aggregate intrinsic value, vested and exercisable | $	$ 5,320,942