Other comprehensive income (loss) - Change in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Foreign Currency Translation
Beginning Balance	$ (68,548)	$ (61,980)	$ (64,528)	$ (56,735)
Other comprehensive income (loss)	165	6,001	(3,855)	756
Net change	165	6,001	(3,855)	756
Ending Balance	(68,383)	(55,979)	(68,383)	(55,979)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(115,632)	(141,327)	(117,894)	(144,335)
Amounts reclassified from accumulated other comprehensive loss for amortization of net losses	2,261	3,008	4,522	6,016
Net change	2,261	3,008	4,522	6,016
Ending Balance		(138,319)		(138,319)
Unrealized net holding losses on debt securities
Beginning Balance	(1,749,006)	(2,098,518)	(1,713,109)	(2,323,903)
Other comprehensive income (loss)	16,941	(69,941)	(54,163)	121,811
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	35,537	34,322	70,745	67,955
Net change	52,478	(35,619)	16,582	189,766
Ending Balance		(2,134,137)		(2,134,137)
Unrealized net 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	$ (1,878,282)	$ (2,328,435)	$ (1,878,282)	$ (2,328,435)	$ (1,895,531)