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Share-based Compensation - Estimated Weighted Average Grant Date Fair Value Per Share (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Estimated weighted average grant date fair value per share    
Expected term 5 years 9 months 18 days 5 years 8 months 12 days
Expected volatility 73.00% 72.00%
Risk-free interest rate 1.99% 1.29%
Expected dividend yield 0.00% 0.00%
Weighted-average grant date fair value per share $ 1.60 $ 2.01