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Stock-Based Compensation - Valuation Assumptions (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Expected volatility 67.13% 59.48%
Expected life (years) 2 years 8 months 2 years 9 months
Risk-free interest rate 2.55% 1.49%