Other comprehensive income (loss) - Change in accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Foreign Currency Translation
Beginning Balance	$ (61,980)	$ (70,165)	$ (56,735)	$ (67,307)
Other comprehensive loss	6,001	5,998	756	3,140
Net change	6,001	5,998	756	3,140
Ending Balance	(55,979)	(64,167)	(55,979)	(64,167)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(141,327)	(155,281)	(144,335)	(158,994)
Other comprehensive income (loss) before reclassification	0	0	0	1,269
Amounts reclassified from accumulated other comprehensive loss for amortization of net losses	3,008	2,444	6,016	4,888
Net change	3,008	2,444	6,016	6,157
Ending Balance	(138,319)	(152,837)	(138,319)	(152,837)
Unrealized net holding (losses) gains on debt securities
Beginning Balance	(2,098,518)	(1,171,950)	(2,323,903)	(96,120)
Other comprehensive (loss) income before reclassifications	(69,941)	(563,420)	121,811	(1,639,250)
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	34,322	0	67,955	0
Net change	(35,619)	(563,420)	189,766	(1,639,250)
Ending Balance	(2,134,137)	(1,735,370)	(2,134,137)	(1,735,370)
Unrealized net losses on cash flow hedges
Beginning Balance	0	158	45	(2,648)
Other comprehensive (loss) income before reclassifications	0	(332)	(19)	2,807
Amounts reclassified from accumulated other comprehensive income (loss)	0	(468)	(26)	(801)
Net change	0	(800)	(45)	2,006
Ending Balance	0	(642)	0	(642)
Accumulated other comprehensive loss, net of tax	$ (2,328,435)	$ (1,953,016)	$ (2,328,435)	$ (1,953,016)	$ (2,524,928)