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Summary of Significant Accounting Policies - Share-Based Compensation (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Stock Based Compensation - Black-Scholes Option-Pricing Model Assumptions:      
Tax benefit from share based compensation $ 0 $ 0  
Stock options      
Stock Based Compensation - Black-Scholes Option-Pricing Model Assumptions:      
Risk-free interest rate 1.40% 1.80% 1.90%
Expected term (in years) 5 years 9 months 15 days 5 years 11 months 18 days 6 years 15 days
Estimated dividend yield 0.00% 0.00% 0.00%
Estimated volatility 48.80% 41.80% 49.90%
ESPP      
Stock Based Compensation - Black-Scholes Option-Pricing Model Assumptions:      
Risk-free interest rate 0.60% 0.30%  
Expected term (in years) 6 months 6 months  
Estimated dividend yield 0.00% 0.00%  
Estimated volatility 37.60% 33.50%