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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)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Stock options      
Weighted average assumptions      
Risk-free interest rate 1.50% 1.80% 1.80%
Volatility 82.70% 78.90% 74.30%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 years 6 years 1 month 6 days 6 years 1 month 6 days
Performance stock options      
Weighted average assumptions      
Risk-free interest rate 1.40% 1.80% 2.10%
Volatility 82.40% 76.70% 69.60%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 5 years 10 months 24 days 6 years 6 years 3 months 18 days
Employee Stock Purchase Plan      
Weighted average assumptions      
Risk-free interest rate 0.50% 0.10% 0.10%
Volatility 93.70% 76.10% 69.80%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 months 6 months 6 months
Minimum      
Weighted average assumptions      
Forfeiture rate 3.00% 3.00%  
Maximum      
Weighted average assumptions      
Forfeiture rate 7.00% 7.00%