Other comprehensive income (loss) - Change in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Foreign Currency Translation
Beginning Balance	$ (64,528)	$ (56,735)
Other comprehensive loss	(4,020)	(5,245)
Net change	(4,020)	(5,245)
Ending Balance	(68,548)	(61,980)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(117,894)	(144,335)
Other comprehensive loss before reclassification	0	0
Amounts reclassified from accumulated other comprehensive loss for amortization of net losses	2,262	3,008
Net change	2,262	3,008
Ending Balance	(115,632)	(141,327)
Unrealized net holding losses on debt securities
Beginning Balance	(1,713,109)	(2,323,903)
Other comprehensive income (loss) before reclassifications	(71,104)	191,752
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	35,207	33,633
Net change	(35,897)	225,385
Ending Balance	(1,749,006)	(2,098,518)
Unrealized net gains on cash flow hedges
Beginning Balance	0	45
Other comprehensive (loss) income before reclassifications	0	(19)
Amounts reclassified from accumulated other comprehensive gains	0	(26)
Net change	0	(45)
Ending Balance	0	0
Total	$ (1,933,186)	$ (2,301,825)	$ (1,895,531)