Stock-based Awards - Stock Options (Details) (Options to purchase common stock, USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Options to purchase common stock
Number of Options
Outstanding (in shares)	18,363,144
Granted (in shares)	10,746,379
Exercised (in shares)	(1,878,681)
Canceled/forfeited (in shares)	(802,368)
Outstanding (in shares)	26,428,474	18,363,144
Vested and expected to vest (in shares)	25,379,923
Exercisable (in shares)	18,469,792
Weighted-Average Exercise Price
Outstanding (in dollars per share)	$ 4.89
Granted (in dollars per share)	$ 15.84
Exercised (in dollars per share)	$ 1.07
Canceled/forfeited (in dollars per share)	$ 9.38
Outstanding (in dollars per share)	$ 9.48	$ 4.89
Vested and expected to vest (in dollars per share)	$ 9.43
Exercisable (in dollars per share)	$ 8.88
Additional disclosures
Weighted Average Remaining Contractual Life, Outstanding	7 years 9 months	7 years 2 months 1 day
Weighted Average Remaining Contractual Life, Vested and expected to vest	7 years 8 months 9 days
Weighted Average Remaining Contractual Life, Exercisable	7 years 1 month 28 days
Remaining compensation expense for unvested awards	$ 67.6
Recognition period	2 years 10 months 24 days