Stock‑Based Compensation (Assumptions) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Valuation of Stock Option Grants to Employees
Expected volatility, minimum	69.70%	73.30%	69.70%	73.10%
Expected volatility, maximum	71.90%	82.80%	78.80%	87.30%
Expected dividend rate	0.00%	0.00%	0.00%	0.00%
Risk‑free interest rate, minimum	1.65%	1.83%	1.56%	1.65%
Risk‑free interest rate, maximum	1.80%	2.04%	1.96%	2.09%
Minimum
Valuation of Stock Option Grants to Employees
Expected term	5 years 8 months 12 days	4 years 10 months 28 days	5 years 7 months 6 days	4 years 10 months 24 days
Maximum
Valuation of Stock Option Grants to Employees
Expected term	6 years 1 month 6 days	5 years 10 months 6 days	6 years 6 months	6 years 3 months 18 days