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Share-Based Compensation - Black Scholes Pricing Model Assumptions used to Estimate Fair Value of Options Granted (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-Based Compensation    
Average risk-free rate of return 2.78% 1.93%
Weighted average expected option life 6 years 1 month 6 days 6 years 1 month 6 days
Estimated volatility 57.60% 68.80%
Average dividend yield 0.00% 0.00%