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Fair Value Measurements - Valuation Technique (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Derivative Liabilities    
Valuation technique for assets measured and recorded at fair value    
Expected dividend yield (as a percent) 0.00%  
Minimum    
Valuation technique for assets measured and recorded at fair value    
Risk-free interest rate (as a percent) 1.64% 1.01%
Volatility (as a percent) 18.40% 62.00%
Expected average term 4 days 4 months 21 days
Minimum | Equity Instruments    
Valuation technique for assets measured and recorded at fair value    
Risk-free interest rate (as a percent) 2.72% 2.30%
Volatility (as a percent) 75.00% 85.00%
Expected average term 8 years 6 months 4 days 9 years 6 months 4 days
Maximum    
Valuation technique for assets measured and recorded at fair value    
Risk-free interest rate (as a percent) 2.56% 2.01%
Volatility (as a percent) 81.69% 108.77%
Expected average term 1 year 6 months 11 days 5 years 2 months 23 days
Maximum | Equity Instruments    
Valuation technique for assets measured and recorded at fair value    
Risk-free interest rate (as a percent) 2.99% 2.36%
Volatility (as a percent) 85.00% 90.00%
Expected average term 9 years 3 months 18 days 10 years
Convertible Senior Notes    
Valuation technique for assets measured and recorded at fair value    
Fair value of Convertible Senior Notes $ 60.6  
Convertible Senior Notes | Level 3    
Valuation technique for assets measured and recorded at fair value    
Volatility (as a percent) 45.00%