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Stock-based compensation - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Minimum    
Stock-based compensation    
Vested and expected to vest range based on shareholder return 50.00%  
Maximum    
Stock-based compensation    
Vested and expected to vest range based on shareholder return 150.00%  
Restricted Stock Award | Directors    
Stock-based compensation    
Stock granted 0 1,052
Restricted Stock Expense $ 0 $ 100,000
Incentive award, tax benefit   $ 6,000
Restricted Stock Units (RSUs)    
Stock-based compensation    
Stock-Based Compensation, vesting rights Unless otherwise stated in an agreement, the compensation cost associated with the shares of restricted stock is determined based on a two-prong vesting schedule. The first part is vested ratably over five years commencing at the date of grant (“the graduated vesting portion”) and the second part is vested at termination of employment after attainment of 55 years of age and 10 years of service (“the retirement vesting portion”). The graduated vesting portion is accelerated at termination of employment after attaining 55 years of age and 10 years of service. The vesting schedule for restricted shares granted on or after 2014 was modified as follows, the graduated vesting portion is vested ratably over four years commencing at the date of the grant and the retirement vesting portion is vested at termination of employment after attainment of the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service. The graduated vesting portion is accelerated at termination of employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service.  
Restricted Stock Units (RSUs) | Executive Officers    
Stock-based compensation    
Stock granted 87,706 84,590
Restricted Stock Expense $ 3,600,000 $ 3,800,000
Incentive award, tax benefit 400,000 $ 400,000
Fair Market Value of stock vested at vesting date 8,700,000  
Windfall Net Of Shortfall 600,000  
Restricted Stock Units (RSUs) | Executive Officers | Share Vesting On Grant Date    
Stock-based compensation    
Fair Market Value of stock vested at vesting date $ 7,000,000  
Restricted Stock Units (RSUs) | Directors    
Stock-based compensation    
Stock granted 298 27,449
Restricted Stock Expense $ 18,000  
Incentive award, tax benefit 2,000  
Fair Market Value of stock vested at vesting date 18,000  
Performance Based Shares | Executive Officers    
Stock-based compensation    
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management $ 9,800,000  
Stock granted 64,815 65,369
Restricted Stock Expense $ 2,500,000 $ 3,600,000
Incentive award, tax benefit $ 200,000 $ 300,000
Stock-Based Compensation, vesting rights The number of shares that will ultimately vest ranges from 50% to a 150% of target based on both market (TSR) and performance (EPS and ROA) conditions. The performance shares vest at the end of the three-year performance cycle. If a participant terminates employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service, the performance shares shall continue outstanding and vest at the end of the performance cycle.  
Performance Based Shares | Executive Officers | Weighted Average    
Stock-based compensation    
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management, period of recognition (in years) 2 years 3 months 10 days