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Stock-based Compensation - Assumptions (Details) - Employees and directors - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Assumptions        
Risk free interest rate 2.90% 1.90% 2.60% 2.10%
Expected term (in years) 6 years 3 months 6 years 3 months 6 years 3 months 6 years 3 months
Expected volatility 77.30% 78.00% 79.80% 77.90%
Unrecognized stock-based compensation expense $ 51.8   $ 51.8  
Period for recognition     2 years 6 months 11 days