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Stock-Based Compensation - Assumptions Using Black-Scholes for Estimated Fair Value of Stock Options (Details) - Employee Stock Option
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Dividend yield 0.00% 0.00% 0.00%
Expected volatility (weighted-average) 103.00% 75.00% 85.00%
Expected term (in years) 5 years 5 months 16 days 6 years 16 days 5 years 9 months 18 days
Minimum      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Risk-free interest rate 0.93% 1.35% 1.42%
Expected volatility (range) 80.00% 73.00% 81.00%
Maximum      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Risk-free interest rate 1.89% 2.20% 2.10%
Expected volatility (range) 1.00% 77.00% 86.00%