Stock‑Based Compensation (Assumptions) (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee and non-employee stock options
Stock Based Compensation
Options granted (in shares)	2,181
Employee stock options
Valuation of Stock Option Grants to Employees
Expected term			6 years
Expected volatility, minimum	69.70%	73.40%	63.70%
Expected volatility, maximum	78.80%	87.00%	85.70%
Expected dividend rate	0.00%	0.00%	0.00%
Risk‑free interest rate, minimum	1.56%	1.65%	0.44%
Risk‑free interest rate, maximum	2.32%	2.04%	2.86%
Employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term	5 years 7 months 6 days	4 years 10 months 28 days
Employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term	10 years	7 years 22 days
Non-employee stock options
Stock Based Compensation
Options granted (in shares)	200
Options unvested (in shares)	100
Valuation of Stock Option Grants to Employees
Expected term			8 years 4 months 24 days
Expected volatility, minimum	70.20%	71.90%	75.60%
Expected volatility, maximum	75.40%	80.20%	81.50%
Expected dividend rate	0.00%	0.00%	0.00%
Risk‑free interest rate, minimum	1.74%	1.41%	1.44%
Risk‑free interest rate, maximum	2.24%	2.61%	2.51%
Non-employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term	4 years 10 months 24 days	4 years 4 months 24 days
Non-employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term	9 years 9 months 18 days	10 years