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Share-Based Employee Compensation - Additional Information (Details)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
$ / shares
shares
Sep. 30, 2016
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
plan
$ / shares
shares
Sep. 30, 2016
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
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 $ 4,181 $ 2,265 $ 9,551 $ 6,800  
Stock Options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of stock options granted (in shares) | shares   0 0  
Compensation expense related to stock options 9 $ 8 $ 25 $ 137  
Total unrecognized compensation expense related to non-vested shares granted 23   $ 23    
Number of years compensation expense to be recognized over     8 months 1 day    
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 $ 10,067   $ 10,067    
Number of years compensation expense to be recognized over     2 years 7 months 24 days    
Number of stock units granted (in shares) | shares 155,000 126,000 680,000 665,000  
Stock units, weighted average grant-date fair value (usd per share) | $ / shares $ 12.93 $ 11.49 $ 10.94 $ 9.20  
Compensation expense related to restricted stocks $ 1,430 $ 1,055 $ 4,090 $ 3,157  
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 $ 10,750   $ 10,750    
Number of years compensation expense to be recognized over     1 year 4 months 28 days    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years    
Number of stock units granted (in shares) | shares 92,000 0 462,000 420,000  
Stock units, weighted average grant-date fair value (usd per share) | $ / shares $ 12.98   $ 10.68 $ 8.61  
Award requisite service period     3 years 3 years  
Cost (reversal) of employee share-based compensation included in income, before income tax $ 2,742 $ 1,073 $ 5,468 $ 3,170  
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%  
Stock Appreciation Rights (SARs)          
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    
Number of stock units granted (in shares) | shares 0   0  
Cost (reversal) of employee share-based compensation included in income, before income tax   $ 129 $ (32) $ 336  
Accrued compensation expense         $ 224