Other comprehensive income (loss) - Change in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Foreign Currency Translation
Beginning Balance	$ (68,383)	$ (55,979)	$ (64,528)	$ (56,735)
Other comprehensive income (loss)	615	(976)	(3,240)	(220)
Net change	615	(976)	(3,240)	(220)
Ending Balance	(67,768)	(56,955)	(67,768)	(56,955)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(113,371)	(138,319)	(117,894)	(144,335)
Amounts reclassified from accumulated other comprehensive loss for amortization of net losses	2,261	3,009	6,784	9,025
Net change	2,261	3,009	6,784	9,025
Ending Balance	(111,110)	(135,310)	(111,110)	(135,310)
Unrealized net holding losses on debt securities
Beginning Balance	(1,696,528)	(2,134,137)	(1,713,109)	(2,323,903)
Other comprehensive income (loss)	326,148	(242,567)	271,985	(120,756)
Amounts reclassified from accumulated other comprehensive loss for amortization of net unrealized losses of debt securities transferred from available-for-sale to held-to-maturity	36,264	35,027	107,008	102,982
Net change	362,412	(207,540)	378,993	(17,774)
Ending Balance	(1,334,116)	(2,341,677)	(1,334,116)	(2,341,677)
Unrealized net (losses) gains on cash flow hedges
Beginning Balance	0	0	0	45
Other comprehensive (loss) income before reclassifications	0	0	0	(19)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	26
Net change	0	0	0	(45)
Ending Balance	0	0	0	0
Total	$ (1,512,994)	$ (2,533,942)	$ (1,512,994)	$ (2,533,942)	$ (1,895,531)