Other comprehensive income (loss) - Disclosure of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Foreign Currency Translation
Beginning Balance	$ (56,783)	$ (49,936)
Other comprehensive income (loss) before reclassifications	(1,818)	(1,238)
Net change	(1,818)	(1,238)
Ending Balance	(58,601)	(51,174)
Adjustment of pension and postretirement benefit plans
Beginning Balance	(202,816)	(203,836)
Amounts reclassified from accumulated other comprehensive income (loss) for amortization of net losses	3,351	3,673
Net change	3,351	3,673
Ending Balance	(199,465)	(200,163)
Unrealized net holding gains (losses) on debt securities
Beginning Balance	92,155	(173,811)
Other comprehensive (loss) income before reclassifications	381,776	101,403
Net change	381,776	101,403
Ending Balance	473,931	(72,408)
Unrealized net (losses) gain on cash flow hedges
Beginning Balance	(2,494)	(391)
Reclassification to retained earnings due to cumulative effect of accounting change	0	(50)
Other comprehensive income (loss) before reclassification	(3,589)	(437)
Amounts reclassified from other accumulated other comprehensive income (loss)	841	644
Net change	(2,748)	157
Ending Balance	(5,242)	(234)
Accumulated other comprehensive (loss) income	$ 210,623	$ (323,979)	$ (169,938)