Equity Award Plan Activities - Option Activities (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Shares Underlying Outstanding Options
Outstanding, beginning balance (in shares)	5,899
Options granted (in shares)	74
Options exercised (in shares)	(613)
Options canceled (in shares)	(8)
Outstanding, ending balance (in shares)	5,352	5,899
Vested and exercisable (in shares)	2,762
Weighted- Average Exercise Price per Share
Outstanding, beginning balance (in dollars per share)	$ 37.09
Options granted (in dollars per share)	225.25
Options exercised (in dollars per share)	28.71
Options canceled (in dollars per share)	35.06
Outstanding, ending balance (in dollars per share)	40.64	$ 37.09
Vested and exercisable (in dollars per share)	$ 26.11
Weighted- Average Remaining Contractual Term (Years) and Aggregate Intrinsic Value of Stock Options
Weighted-average remaining contractual term of stock options outstanding	5 years 26 days	5 years 2 months 12 days
Weighted-average remaining contractual term of stock options vested and exercisable	4 years 6 months 4 days
Aggregate intrinsic value of stock options outstanding	$ 1,465,345	$ 1,027,741
Aggregate intrinsic value of stock options outstanding, vested and exercisable	$ 796,325