Other comprehensive income (loss) - Disclosure of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Foreign Currency Translation
Beginning Balance	$ (67,959)	$ (69,458)	$ (71,254)	$ (56,783)
Other comprehensive income (loss) before reclassifications	(1,265)	(2,035)	2,030	(14,710)
Net change	(1,265)	(2,035)	2,030	(14,710)
Ending Balance	(69,224)	(71,493)	(69,224)	(71,493)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(188,572)	(196,114)	(195,056)	(202,816)
Amounts reclassified from accumulated other comprehensive income (loss) for amortization of net losses	3,242	3,351	9,726	10,053
Net change	3,242	3,351	9,726	10,053
Ending Balance	(185,330)	(192,763)	(185,330)	(192,763)
Unrealized net holding gains (losses) on debt securities
Beginning Balance	163,982	489,105	460,900	92,155
Other comprehensive (loss) income before reclassifications	(46,956)	(5,243)	(343,874)	391,707
Amounts reclassified from accumulated other comprehensive income (loss) for (gains) loss on securities	(18)	(35)	(18)	(35)
Net change	(46,974)	(5,278)	(343,892)	391,672
Ending Balance	117,008	483,827	117,008	483,827
Unrealized net (losses) gain on cash flow hedges
Beginning Balance	(3,142)	(4,184)	(4,599)	(2,494)
Other comprehensive income (loss) before reclassification	(465)	(1,085)	662	(5,067)
Amounts reclassified from other accumulated other comprehensive income (loss)	1,058	1,114	1,388	3,406
Net change	593	29	2,050	(1,661)
Ending Balance	(2,549)	(4,155)	(2,549)	(4,155)
Accumulated other comprehensive (loss) income	$ (140,095)	$ 215,416	$ (140,095)	$ 215,416	$ 189,991