Other comprehensive income (loss) - Change in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Foreign Currency Translation
Beginning Balance	$ (64,528)	$ (56,735)	$ (67,307)
Other comprehensive (loss) income	(6,837)	(7,793)	10,572
Net change	(6,837)	(7,793)	10,572
Ending Balance	(71,365)	(64,528)	(56,735)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(117,893)	(144,335)	(158,994)
Other comprehensive income before reclassifications	14,157	14,408	4,882
Amounts reclassified from accumulated other comprehensive loss for amortization of net losses	9,044	12,034	9,777
Net change	23,201	26,442	14,659
Ending Balance	(94,692)	(117,893)	(144,335)
Unrealized net holding (losses) gains on debt securities
Beginning Balance	(1,713,110)	(2,323,903)	(96,120)
Other comprehensive income (loss) before reclassifications	74,277	472,487	(2,261,097)
Amounts reclassified from accumulated other comprehensive (loss) income for gains on securities	0	0	0
Amounts reclassified from accumulated other comprehensive (loss) income for amortization of net unrealized losses of debt securities transferred from available-for-sale to held-to-maturity	143,650	138,306	33,314
Net change	217,927	610,793	(2,227,783)
Ending Balance	(1,495,183)	(1,713,110)	(2,323,903)
Unrealized net gains (losses) on cash flow hedges
Beginning Balance	0	45	(2,648)
Other comprehensive (loss) income before reclassifications	0	(19)	3,107
Amounts reclassified from accumulated other comprehensive (loss) income for gains on securities	0	(26)	(414)
Net change	0	(45)	2,693
Ending Balance	0	0	45
Total	$ (1,661,240)	$ (1,895,531)	$ (2,524,928)