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Share-based compensation plans - Additional Information (Details)
$ / shares in Units, $ in Thousands
12 Months Ended 48 Months Ended
Dec. 31, 2019
USD ($)
installment
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2014
Dec. 31, 2019
USD ($)
installment
$ / shares
shares
Apr. 17, 2019
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Maximum number of shares of common stock issued (in shares) | shares           12,000,000
Shares available for grant (in shares) | shares 10,765,398       10,765,398  
Compensation cost for share-based payment arrangements $ 14,334 $ 10,730 $ 13,488      
Related tax benefit recognized in net income 3,500 2,678 5,058      
Additional net excess tax benefit realized 3,520 3,528 2,453      
Noncash stock-based compensation associated performance contingent restricted stock units 5,171 4,725 3,896      
Compensation deferrals in current year $ 2,585 $ 1,452 $ 2,850      
Stock Appreciation Rights (SARs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Expected life of SARs 6 years 6 years 6 years      
Total unrecognized compensation cost related to nonvested awards $ 2,413       $ 2,413  
Weighted-average period 24 months          
Aggregate intrinsic value of options and SARs exercised $ 11,836 $ 9,029 $ 3,786      
Weighted-average fair value of awards granted (usd per share) | $ / shares $ 8.30 $ 6.55 $ 7.29      
Weighted average remaining contractual life for SAR's, outstanding 7 years 6 months          
Weighted average remaining contractual life for SAR's, exercisable 6 years          
Aggregate intrinsic value for SAR's, outstanding $ 13,375       13,375  
Aggregate intrinsic value for SAR's, exercisable $ 8,931       $ 8,931  
Fair market value of the Company’s stock used to calculate intrinsic value (usd per share) | $ / shares $ 61.72       $ 61.72  
Stock Appreciation Rights (SARs) | After Two Thousand Fifteen            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Minimum vesting period         3 years  
Stock Appreciation Rights (SARs) | After Two Thousand Six            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Minimum vesting period 2 years          
Stock Appreciation Rights (SARs) | After Two Thousand Fourteen            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Minimum vesting period         3 years  
Term of award 10 years          
Stock Options and Stock Appreciation Rights Sars            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Compensation cost for share-based payment arrangements $ 3,227 $ 2,415 $ 3,719      
Performance Shares            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Total unrecognized compensation cost related to nonvested awards $ 6,806       $ 6,806  
Weighted-average period 19 months          
Performance period 3 years          
Number of common shares to be converted into for each share upon conversion 1       1  
Granted, weighted-average grant date fair value (usd per share) | $ / shares $ 56.04 $ 46.33 $ 50.11      
Performance Shares | 2019 PCSU            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Total performance contingent restricted stock units vested , minimum (in shares) | shares 0          
Total performance contingent restricted stock units, maximum (in shares) | shares 228,650          
Performance Shares | 2018 PCSU            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Total performance contingent restricted stock units vested , minimum (in shares) | shares 0          
Total performance contingent restricted stock units, maximum (in shares) | shares 253,962          
Performance Shares | 2017 PCSU            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vested units in period (in shares) | shares 84,522          
Fair value of vested units $ 5,217          
Performance Shares | 2016 PCSU            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vested units in period (in shares) | shares   132,534        
Fair value of vested units   $ 7,042        
Performance Shares | 2015 PCSU            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vested units in period (in shares) | shares     135,695      
Fair value of vested units     $ 7,211      
Restricted Stock Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vested units in period (in shares) | shares 0          
Granted, weighted-average grant date fair value (usd per share) | $ / shares $ 57.76          
Restricted Stock Awards | Executives and Directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Minimum vesting period 3 years          
Total unrecognized compensation cost related to nonvested awards $ 3,768       $ 3,768  
Weighted-average period 33 months          
Fair value of vested units $ 3,217 $ 6,900 $ 2,790      
Granted, weighted-average grant date fair value (usd per share) | $ / shares $ 57.76 $ 48.36 $ 51.68      
Noncash stock-based compensation associated performance contingent restricted stock units $ 3,351 $ 2,138 $ 3,554      
Restricted Stock Awards | Share-based Compensation Award, Tranche One | Executives and Directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage 33.33%     33.33%    
Restricted Stock Awards | Share-based Compensation Award, Tranche Two | Executives and Directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage 33.33%     33.33%    
Restricted Stock Awards | Share-based Compensation Award, Tranche Three | Executives and Directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage 33.33%     33.33%    
Restricted Stock Awards | Share-based Compensation Award, Tranche Four | Executives and Directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage       33.33%    
Restricted Stock Awards | Share-based Compensation Award, Tranche Five | Executives and Directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage       33.33%    
Deferred Stock Equivalent Units | Share-based Compensation Award, Tranche One | Non-employee directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of annual distribution installments | installment 1       1  
Deferred Stock Equivalent Units | Share-based Compensation Award, Tranche Two | Non-employee directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of annual distribution installments | installment 3       3  
Deferred Stock Equivalent Units | Share-based Compensation Award, Tranche Three | Non-employee directors            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of annual distribution installments | installment 5       5