Other comprehensive income (loss) - Reclassification out of accumulated other comprehensive loss (Details) - Reclassifications Out of Accumulated Other Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reclassifications Out of Accumulated Other Comprehensive Loss
Net of tax	$ (37,469)	$ (36,615)
Adjustment of pension and postretirement benefit plans
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(3,618)	(4,813)
Income tax benefit (expense)	1,356	1,805
Net of tax	(2,262)	(3,008)
Adjustment of pension and postretirement benefit plans | Other operating expenses
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(3,618)	(4,813)
Unrealized holding losses on debt securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(44,009)	(42,040)
Income tax benefit (expense)	8,802	8,407
Net of tax	(35,207)	(33,633)
Unrealized holding losses on debt securities | Investment securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(44,009)	(42,040)
Unrealized net gains on cash flow hedges
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	0	41
Income tax benefit (expense)	0	(15)
Net of tax	0	26
Unrealized net gains on cash flow hedges | Forward contracts | Mortgage banking activities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	$ 0	$ 41