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Stock-based compensation - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Executive Officers          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management $ 13,900   $ 13,900    
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management, period of recognition (in years)     1 year 7 months 24 days    
Fair value of stocks vested at grant date     $ 16,900    
Fair Market Value of stock vested     23,200    
Windfall Net Of Shortfall     $ 2,300    
Directors          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Stock granted     24,279   39,104
Restricted Stock Expense (Benefit) 100 $ 100 $ 2,200 $ 2,100  
Incentive award, tax benefit (expense) $ 21 $ 16 400 $ 400  
Fair Market Value of stock vested     $ 2,200    
Restricted Stock Units (RSUs)          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Stock-Based Compensation, vesting rights     Unless otherwise stated in an agreement, the compensation cost associated with the shares of restricted stock granted prior to 2021 was determined based on a two-prong vesting schedule. These grants include ratable vesting over five or four years commencing at the date of grant (“the graduated vesting portion”) with a portion vested at termination of employment after attainment of 55 years of age and 10 years of service or 60 years of age and 5 years of service (“the retirement vesting portion”). 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 granted on or after 2021 have ratable vesting in equal annual installments over a period of 4 years or 3 years, depending on the classification of the employee. The vesting schedule 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          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Stock granted 928 0 176,519 200,303  
Restricted Stock Expense (Benefit) $ 1,800 $ 2,000 $ 12,300 $ 9,700  
Incentive award, tax benefit (expense) $ 400 $ 400 $ 2,000 $ 1,500  
Restricted Stock Units (RSUs) | Directors          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Stock granted 1,281 1,384 22,887 35,412  
Performance Based Shares | Minimum          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Vested and expected to vest range based on shareholder return 50.00%   50.00%    
Performance Based Shares | Maximum          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Vested and expected to vest range based on shareholder return 150.00%   150.00%    
Performance Based Shares | Executive Officers          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Stock granted 0 0 65,225 51,355  
Restricted Stock Expense (Benefit) $ (500) $ 100 $ 3,500 $ 3,600  
Incentive award, tax benefit (expense) $ (32) $ 8 $ 200 $ 100  
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 (ROATCE) 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.    
Unrestricted Stock | Directors          
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]          
Stock granted 0 0 1,392 2,300