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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 (Detail)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Stock options      
Weighted average assumptions      
Risk-free interest rate 1.80% 1.80% 1.90%
Volatility 78.90% 74.30% 72.40%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 years 1 month 6 days 6 years 1 month 6 days 6 years 1 month 6 days
Performance stock options      
Weighted average assumptions      
Risk-free interest rate 1.80% 2.10% 0.00%
Volatility 76.70% 69.60% 0.00%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 years 6 years 3 months 18 days 0 years
Employee Stock Purchase Plan      
Weighted average assumptions      
Risk-free interest rate 0.10% 0.10% 0.10%
Volatility 76.10% 69.80% 58.80%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 months 6 months 6 months
Minimum      
Weighted average assumptions      
Forfeiture rate, as a percent 3.00% 3.00%  
Maximum      
Weighted average assumptions      
Forfeiture rate, as a percent 8.00% 8.00%