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Share-Based Compensation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares received per restricted stock unit (in shares)     1    
Total compensation cost related to nonvested awards, unrecognized $ 48,924   $ 48,924   $ 32,812
Weighted-average period to recognize compensation cost (in years)     2 years    
Aggregate intrinsic value for outstanding SARs and RSUs $ 99,878   $ 99,878    
Aggregate intrinsic value for vested SARs and RSUs     $ 35,996    
Weighted-average remaining contractual life for outstanding SARs and RSUs (in years)     5 years    
Share-based compensation     $ 9,035 $ 8,086  
Outstanding options to purchase common shares not included in dilutive shares (in shares) 2,098,000 1,968,000 1,445,000 1,697,000  
Restricted Stock Units (RSUs) [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Grants in period (in shares)     360,000    
Minimum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based payment award granted vesting period range (in years)     1 year    
Share-based payment, awards exercise period (in years and months)     12 months    
Maximum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based payment award granted vesting period range (in years)     5 years    
Share-based payment, awards exercise period (in years and months)     10 years