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Stock-Based Compensation - Fair Value Assumptions for Stock Options (Details) - Stock options
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 1.30% 1.80% 1.60%
Dividend yield 0.00% 0.00% 0.00%
Volatility 78.80% 74.50% 80.40%
Expected term (in years) 2 years 7 months 6 days 5 years 5 years