Other comprehensive income (loss) - Disclosure of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Foreign Currency Translation
Beginning Balance	$ (69,458)	$ (52,378)	$ (56,783)	$ (49,936)
Other comprehensive income (loss) before reclassifications	(2,035)	155	(14,710)	(2,287)
Net change	(2,035)	155	(14,710)	(2,287)
Ending Balance	(71,493)	(52,223)	(71,493)	(52,223)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(196,114)	(196,491)	(202,816)	(203,836)
Amounts reclassified from accumulated other comprehensive income (loss) for amortization of net losses	3,351	3,673	10,053	11,018
Net change	3,351	3,673	10,053	11,018
Ending Balance	(192,763)	(192,818)	(192,763)	(192,818)
Unrealized net holding gains (losses) on debt securities
Beginning Balance	489,105	58,044	92,155	(173,811)
Other comprehensive (loss) income before reclassifications	(5,243)	50,970	391,707	282,825
Amounts reclassified from accumulated other comprehensive (loss) income for gains on securities	(35)	16	(35)	16
Net change	(5,278)	50,986	391,672	282,841
Ending Balance	483,827	109,030	483,827	109,030
Unrealized net (losses) gain on cash flow hedges
Beginning Balance	(4,184)	(388)	(2,494)	(391)
Reclassification to retained earnings due to cumulative effect of accounting change	0	0	0	(50)
Other comprehensive income (loss) before reclassification	(1,085)	(3,201)	(5,067)	(4,349)
Amounts reclassified from other accumulated other comprehensive income (loss)	1,114	720	3,406	1,921
Net change	29	(2,481)	(1,661)	(2,478)
Ending Balance	(4,155)	(2,869)	(4,155)	(2,869)
Accumulated other comprehensive (loss) income	$ 215,416	$ (138,880)	$ 215,416	$ (138,880)	$ (169,938)