Stock-based compensation - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
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	$ 13,600,000		$ 13,600,000
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 6 days
Fair value of stocks vested at grant date			$ 11,300,000
Fair Market Value of stock vested at vesting date			14,100,000
Windfall Net Of Shortfall			$ 1,000,000.0
Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted			37,712		25,321
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. The first part is vested ratably over five or four 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 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 will vest ratably 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	0	1,888	200,303	137,934
Restricted Stock Expense (Benefit)	$ 2,000,000.0	$ 1,500,000	$ 9,700,000	$ 8,900,000
Incentive award, tax benefit (expense)	$ 400,000	$ 300,000	$ 1,500,000	$ 1,500,000
Restricted Stock Units (RSUs) | Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	1,384	857	35,412	24,409
Restricted Stock Expense (Benefit)	$ 100,000	$ 100,000	$ 2,100,000	$ 1,900,000
Incentive award, tax benefit (expense)	$ 16,000	$ 25,000	400,000	$ 400,000
Fair Market Value of stock vested at vesting date			$ 2,100,000
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	51,355	56,857
Restricted Stock Expense (Benefit)	$ 100,000	$ 300,000	$ 3,600,000	$ 4,300,000
Incentive award, tax benefit (expense)	$ 8,000	$ 13,000	$ 100,000	$ 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 (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		2,300