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Share-Based Employee Compensation - Additional Information (Details)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Jun. 30, 2017
USD ($)
$ / shares
shares
Jun. 30, 2018
USD ($)
plan
$ / shares
shares
Jun. 30, 2017
USD ($)
$ / shares
shares
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shareholder approved stock plans | plan     2    
Cost (reversal) of employee share-based compensation included in income, before income tax $ 3,464 $ 2,185 $ 6,464 $ 5,370  
Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Total unrecognized compensation expense related to non-vested shares granted $ 12,642   $ 12,642    
Number of years compensation expense to be recognized over     2 years 8 months 1 day    
Number of stock units granted (in shares) | shares 59 60 419 526  
Stock units, weighted average grant-date fair value (usd per share) | $ / shares $ 17.72 $ 12.04 $ 15.22 $ 10.35  
Compensation expense related to restricted stocks $ 1,463 $ 1,439 $ 2,850 $ 2,660  
Restricted Stock Units (RSUs) | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     1 year    
Restricted Stock Units (RSUs) | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years    
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Total unrecognized compensation expense related to non-vested shares granted $ 14,087   $ 14,087    
Number of years compensation expense to be recognized over     1 year 7 months 17 days    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years    
Number of stock units granted (in shares) | shares 0 0 307 370  
Stock units, weighted average grant-date fair value (usd per share) | $ / shares     $ 14.80 $ 10.10  
Award requisite service period     3 years 3 years  
Cost (reversal) of employee share-based compensation included in income, before income tax $ 1,995 $ 737 $ 3,599 $ 2,725  
Performance Shares | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award requisite service period     1 year    
Shares awarded as a percentage of granted     0.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Target Award Shares, Percent     50.00% 50.00% 50.00%
Performance Shares | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award requisite service period     3 years    
Shares awarded as a percentage of granted     200.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Target Award Shares, Percent     80.00% 80.00% 80.00%