Other comprehensive income (loss) - Reclassification out of accumulated other comprehensive loss (Details) - Reclassifications Out of Accumulated Other Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reclassifications Out of Accumulated Other Comprehensive Loss
Net of tax	$ (152,694)	$ (150,314)	$ (42,677)
Adjustment of pension and postretirement benefit plans
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(14,471)	(19,253)	(15,644)
Income tax benefit (expense)	5,427	7,219	5,867
Net of tax	(9,044)	(12,034)	(9,777)
Adjustment of pension and postretirement benefit plans | Other operating expenses
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(14,471)	(19,253)	(15,644)
Unrealized net holding (losses) gains on debt securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(179,563)	(172,883)	(41,642)
Income tax benefit (expense)	35,913	34,577	8,328
Net of tax	(143,650)	(138,306)	(33,314)
Unrealized net holding (losses) gains on debt securities | Investment securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(179,563)	(172,883)	(41,642)
Unrealized net gains (losses) on cash flow hedges
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	0	41	960
Income tax benefit (expense)	0	(15)	(546)
Net of tax	0	26	414
Unrealized net gains (losses) on cash flow hedges | Forward contracts | Mortgage banking activities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	0	41	1,458
Unrealized net gains (losses) on cash flow hedges | Interest rate swap
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	$ 0	$ 0	$ (498)