Equity Award Plan Activities - Option Activity Rollforward (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Number of Shares Underlying Outstanding Options
Outstanding, beginning balance (in shares)	7,024
Options granted (in shares)	82
Options exercised (in shares)	(1,038)
Options canceled (in shares)	(41)
Outstanding, ending balance (in shares)	6,027	7,024
Vested and exercisable (in shares)	2,734
Weighted- Average Exercise Price per Share
Outstanding, beginning balance (in dollars per share)	$ 33.05
Options granted (in dollars per share)	244.20
Options exercised (in dollars per share)	33.07
Options canceled (in dollars per share)	53.44
Outstanding, ending balance (in dollars per share)	35.78	$ 33.05
Vested and exercisable (in dollars per share)	$ 22.39
Weighted- Average Remaining Contractual Term (Years) and Aggregate Intrinsic Value of Stock Options
Weighted-average remaining contractual term of stock options outstanding	5 years 5 months 9 days	6 years 1 month 2 days
Weighted-average remaining contractual term of stock options vested and exercisable	4 years 10 months 2 days
Aggregate intrinsic value of stock options outstanding	$ 1,386,807	$ 1,422,637
Aggregate intrinsic value of stock options outstanding, vested and exercisable	$ 665,576