Income taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Property, plant and equipment	$ (91,207)	$ (102,007)
Intangibles	(134,868)	(178,883)
Leases	(79,332)
Gross deferred tax liabilities	(305,407)	(280,890)
Retiree health benefits	2,405	2,989
Foreign loss carryforwards	58,527	57,581
U.S. Federal loss and credit carryforwards	86,748	86,655
Capital loss carryforwards	2,703	2,757
Employee benefits	87,295	114,872
Leases	79,673
Accrued liabilities and other	63,700	102,349
Gross deferred tax assets	381,051	367,203
Valuation allowance on deferred tax assets	(105,347)	(103,289)
Total deferred taxes, net	$ (29,703)	$ (16,976)