Stock‑Based Compensation (Options) (Details) - Employee and non-employee stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Stock Based Compensation
Shares available for grant, beginning balance	294
Additional shares authorized	4,985
Options granted (in shares)	(1,736)
Options forfeited (in shares)	477
Shares available for grant, end balance	4,020	294
Number of shares
Outstanding, beginning balance (in shares)	8,463
Options granted (in shares)	1,736
Options exercised (in shares)	(323)
Options forfeited (in shares)	(477)
Outstanding, end balance (in shares)	9,399	8,463
Exercisable (in shares)	5,727
Vested and expected to vest (in shares)	8,473
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per shares)	$ 2.28
Granted (in dollars per share)	10.19
Exercised (in dollars per share)	1.76
Forfeited (in dollars per share)	3.49
Outstanding, end balance (in dollars per shares)	3.25	$ 2.28
Exercisable (in dollars per share)	1.70
Vested and expected to vest (in dollars per share)	$ 3.39
Additional disclosures
Weighted average contractual term, options outstanding	8 years 5 months 23 days	8 years 9 months 29 days
Exercisable (in years)	6 years
Vested and expected to vest (in years)	7 years 8 months 16 days
Aggregate intrinsic value, options outstanding	$ 65,607
Aggregate intrinsic value, options exercisable	51,081
Aggregate intrinsic value, vested and expected to vest	$ 60,979