Stock-Based Compensation - Option and RSA Activity Rollforward (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Shares Available for Grant
Outstanding, beginning balance (in shares)	8,941,000
Authorized (in shares)	6,510,000
Options granted (in shares)	(4,521,000)
Options canceled (in shares)	654,000
Early exercised shares repurchased (in shares)	28,000	[1]
Outstanding, ending balance (in shares)	11,612,000		8,941,000
Number of Shares Underlying Outstanding Options and RSAs (in thousands)
Outstanding, beginning balance (in shares)	11,245,000
Options granted (in shares)	4,412,000
Options exercised (in shares)	(1,349,000)
Options canceled (in shares)	(654,000)
Early exercised shares repurchased (in shares)	0	[1]
Outstanding, ending balance (in shares)	13,654,000		11,245,000
Vested and exercisable (in shares)	3,397,000
Vested and expected to vest (in shares)	12,034,000
Weighted- Average Exercise Price per Share
Outstanding, beginning balance (in dollars per share)	$ 6.82
Options granted (in dollars per share)	$ 40.88
Options exercised (in dollars per share)	$ 6.04
Options canceled (in dollars per share)	$ 12.45
Early exercised shares repurchased (in dollars per share)	$ 4.05	[1]
Outstanding, ending balance (in dollars per share)	$ 17.63		$ 6.82
Vested and exercisable (in dollars per share)	$ 5.36
Vested and expected to vest (in dollars per share)	$ 16.12
Weighted- Average Remaining Contractual Term (Years) of Stock Options
Weighted-average remaining contractual term of stock options outstanding	8 years 3 months 18 days		8 years 9 months 18 days
Weighted-average remaining contractual term of stock options vested and exercisable	7 years 4 months 28 days
Weighted-average remaining contractual term of stock options vested and expected to vest	8 years 2 months 27 days
Aggregate intrinsic value of stock options outstanding, beginning of period	$ 254,319
Aggregate intrinsic value of stock options outstanding, end of period	598,775		254,319
Aggregate intrinsic value of stock options outstanding vested and exercisable	188,223
Aggregate intrinsic value of stock options outstanding vested and expected to vest	$ 544,919
Early exercised shares repurchased and retired	2,000

[1] Early exercised shares repurchased includes 2,000 shares that were originally granted out of our 2004 Plan and were subsequently retired.