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Stock- based Compensation - Option Pricing Assumptions (Details)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividend yield 0.00% 0.00% 0.00%
Expected volatility     67.00%
Expected term 6 years 3 months 6 years 3 months 6 years 3 months
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 64.00% 73.00%  
Risk-free interest rate 1.20% 1.60% 1.80%
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 69.00% 75.00%  
Risk-free interest rate 2.40% 2.10% 2.30%
Options granted, maximum term 10 years