Stock-based compensation - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	25,462	39,104	25,321
Employee Stock Option
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	$ 11.6
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 8 months 1 day
Restricted Stock Units (RSUs) | Executive Officers
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	177,249	200,303	137,934
Restricted Stock Expense (Benefit)	$ 14.0	$ 11.5	$ 10.3
Incentive award, tax benefit (expense)	2.4	$ 1.9	$ 1.8
Fair value of stocks vested at grant date	$ 23.3
Stock-Based Compensation, vesting rights	, 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 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 in 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.
Windfall Net Of Shortfall	$ 2.3
Restricted Stock Units (RSUs) | Executive Officers | Share Vesting On Grant Date
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Fair value of stocks vested at grant date	$ 17.1
Restricted Stock Units (RSUs) | Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	24,070	36,804	25,321
Restricted Stock Expense (Benefit)	$ 2.2	$ 2.2	$ 2.0
Incentive award, tax benefit (expense)	0.4	$ 0.4	$ 0.4
Fair value of stocks vested at grant date	$ 2.3
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%
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%
Performance Based Shares | Executive Officers
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	65,225	57,454	56,857
Restricted Stock Expense (Benefit)	$ 3.9	$ 3.5	$ 4.8
Incentive award, tax benefit (expense)	$ 0.3	$ 0.1	$ 0.4
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	1,392	2,300	0