Stock-based Compensation - Assumptions (Details) - Employee and Consultant Options - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employees and directors
Assumptions
Expected volatility	77.80%	78.40%	78.80%
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Risk free interest rate	2.10%	1.50%	1.70%
Employees
Assumptions
Unrecognized stock-based compensation expense	$ 33.4
Period for recognition	2 years 6 months
Non-employees
Assumptions
Expected volatility		76.50%	80.00%
Expected term (in years)		10 years	10 years
Risk free interest rate		1.60%	2.20%