Other comprehensive income (loss) - Disclosure of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Foreign Currency Translation
Beginning Balance	$ (70,685)	$ (58,601)	$ (71,254)	$ (56,783)
Other comprehensive income (loss) before reclassifications	2,726	(10,857)	3,295	(12,675)
Net change	2,726	(10,857)	3,295	(12,675)
Ending Balance	(67,959)	(69,458)	(67,959)	(69,458)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(191,814)	(199,465)	(195,056)	(202,816)
Amounts reclassified from accumulated other comprehensive income (loss) for amortization of net losses	3,242	3,351	6,484	6,702
Net change	3,242	3,351	6,484	6,702
Ending Balance	(188,572)	(196,114)	(188,572)	(196,114)
Unrealized net holding gains (losses) on debt securities
Beginning Balance	90,955	473,931	460,900	92,155
Other comprehensive (loss) income before reclassifications	73,027	15,174	(296,918)	396,950
Net change	73,027	15,174	(296,918)	396,950
Ending Balance	163,982	489,105	163,982	489,105
Unrealized net (losses) gain on cash flow hedges
Beginning Balance	(3,194)	(5,242)	(4,599)	(2,494)
Other comprehensive income (loss) before reclassification	(230)	(393)	1,127	(3,982)
Amounts reclassified from other accumulated other comprehensive income (loss)	282	1,451	330	2,292
Net change	52	1,058	1,457	(1,690)
Ending Balance	(3,142)	(4,184)	(3,142)	(4,184)
Accumulated other comprehensive (loss) income	$ (95,691)	$ 219,349	$ (95,691)	$ 219,349	$ 189,991