Other comprehensive income - Schedule of Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Reclassifications Out of Accumulated Other Comprehensive Loss
Other operating expenses	$ (11,744)	$ (22,117)	$ (25,949)	$ (45,016)
Investment securities	220,352	189,753	421,179	369,912
Income before income tax	323,961	258,324	616,571	480,889
Income tax benefit	(45,747)	(47,884)	(92,683)	(92,947)
Net Income	278,214	210,440	523,888	387,942
Reclassifications Out of Accumulated Other Comprehensive Loss
Reclassifications Out of Accumulated Other Comprehensive Loss
Net Income	(36,521)	(38,414)	(75,433)	(76,083)
Reclassifications Out of Accumulated Other Comprehensive Loss | Adjustment of pension and postretirement benefit plans
Reclassifications Out of Accumulated Other Comprehensive Loss
Other operating expenses	(2,258)	(2,272)	(4,516)	(4,545)
Income before income tax	(2,258)	(2,272)	(4,516)	(4,545)
Income tax benefit	847	852	1,694	1,704
Net Income	(1,411)	(1,420)	(2,822)	(2,841)
Reclassifications Out of Accumulated Other Comprehensive Loss | Unrealized net holding losses on debt securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Investment securities	(43,886)	(46,242)	(90,762)	(91,552)
Income before income tax	(43,886)	(46,242)	(90,762)	(91,552)
Income tax benefit	8,776	9,248	18,151	18,310
Net Income	$ (35,110)	$ (36,994)	$ (72,611)	$ (73,242)