Income taxes - Components of Net Deferred Tax Asset (Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Tax effect of temporary differences related to:
Lease assets	$ 5.2	$ 5.7
Accrued OPEB costs	1.9	2.1
Accrued pension costs	14.3	26.9
Accrued environmental liabilities	15.2	22.6
Capitalized research and development costs	6.3	4.8
Other deductible differences	10.1	10.7
Investments in subsidiaries and affiliates	10.1	10.5
Tax loss and tax credit carryforwards	126.6	119.6
Valuation allowance	(14.1)	(3.5)
Adjusted gross deferred tax assets	175.6	199.4
Netting of items by tax jurisdiction, assets	(121.8)	(132.4)
Net noncurrent deferred tax asset	53.8	67.0
Tax effect of temporary differences related to:
Property and equipment	(69.1)	(62.9)
Lease liabilities	(5.3)	(5.7)
Other taxable differences	(22.6)	(15.3)
Investments in subsidiaries and affiliates	(56.9)	(54.7)
Unrecognized foreign currency gain	(16.8)
Tax on unremitted earnings of non-U.S. subsidiaries	(8.8)	(10.9)
Adjusted gross deferred tax liabilities	(179.5)	(149.5)
Netting of items by tax jurisdiction, liabilities	(121.8)	(132.4)
Net noncurrent deferred tax liability	$ (57.7)	$ (17.1)