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Share-based Compensation - Summary of Activity in Options (Details) - 2015 Management Incentive Plan - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Number of Options        
Outstanding at beginning of period (in shares) 3,486,150 7,783,000 7,783,000  
Granted (in shares) 156,129      
Exercised (in shares) (208,700) (732,000)    
Forfeited or expired (in shares) (40,000)      
Outstanding at end of period (in shares) 3,393,579 7,051,000 3,486,150 7,783,000
Weighted Average Exercise Price Per Share        
Outstanding at beginning of period (in dollars per share) $ 19.00 $ 19.00 $ 19.00  
Granted (in dollars per share) 13.60      
Exercised (in dollars per share) 19.00 19.00    
Outstanding at end of period (in dollars per share) $ 18.75 $ 19.00 $ 19.00 $ 19.00
Weighted Average Remaining Contractual Life        
Weighted Average Remaining Contractual Life 5 years 11 months 26 days 7 years 18 days 6 years 3 months 19 days 7 years 3 months 15 days
Weighted Average Remaining Contractual Life, Granted 4 years 10 months 14 days      
Options Exercisable, Number of Options (in shares) 1,607,829 3,182,000 1,660,400 3,891,500
Options Exercisable, Weighted Average Exercise Price (in dollars per share) $ 18.48 $ 19.00 $ 19.00 $ 19.00