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Common Stock and Stockholders' Equity - Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2017
Stock options        
Weighted average assumptions        
Risk-free interest rate 2.00% 1.50% 1.80%  
Volatility 89.40% 82.70% 78.90%  
Dividend yield 0.00% 0.00% 0.00%  
Expected term (years) 6 years 1 month 6 days 6 years 6 years 1 month 6 days  
Performance stock options        
Weighted average assumptions        
Risk-free interest rate 2.10% 1.40% 1.80%  
Volatility 89.20% 82.40% 76.70%  
Dividend yield 0.00% 0.00% 0.00%  
Expected term (years) 5 years 8 months 12 days 5 years 10 months 24 days 6 years  
Common stock available for future purchase under the 2012 Employee Stock Purchase Plan        
Weighted average assumptions        
Risk-free interest rate 0.90% 0.50% 0.10%  
Volatility 105.60% 93.70% 76.10%  
Dividend yield 0.00% 0.00% 0.00%  
Expected term (years) 6 months 6 months 6 months  
Accounting Standards Update 2016-09 | Additional paid-in capital        
Weighted average assumptions        
Cumulative effect of new accounting principle       $ 0.6