Other comprehensive income (loss) - Change in accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Foreign Currency Translation
Beginning Balance	$ (55,979)	$ (64,167)	$ (56,735)	$ (67,307)
Other comprehensive (loss) income	(976)	7,206	(220)	10,346
Net change	(976)	7,206	(220)	10,346
Ending Balance	(56,955)	(56,961)	(56,955)	(56,961)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(138,319)	(152,837)	(144,335)	(158,994)
Other comprehensive income before reclassification	0	0	0	1,269
Amounts reclassified from accumulated other comprehensive loss for amortization of net losses	3,009	2,444	9,025	7,332
Net change	3,009	2,444	9,025	8,601
Ending Balance	(135,310)	(150,393)	(135,310)	(150,393)
Unrealized net holding losses on debt securities
Beginning Balance	(2,134,137)	(1,735,370)	(2,323,903)	(96,120)
Other comprehensive loss	(242,567)	(781,898)	(120,756)	(2,421,148)
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	35,027	0	102,982	0
Net change	(207,540)	(781,898)	(17,774)	(2,421,148)
Ending Balance	(2,341,677)	(2,517,268)	(2,341,677)	(2,517,268)
Unrealized net (losses) gains on cash flow hedges
Beginning Balance	0	(642)	45	(2,648)
Other comprehensive (loss) income before reclassifications	0	415	(19)	3,222
Amounts reclassified from accumulated other comprehensive income (loss)	0	518	(26)	(283)
Net change	0	933	(45)	2,939
Ending Balance	0	291	0	291
Accumulated other comprehensive loss, net of tax	$ (2,533,942)	$ (2,724,331)	$ (2,533,942)	$ (2,724,331)	$ (2,524,928)