Stockholders' equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 21.8	$ (64.4)	$ (312.1)
Other comprehensive income (loss) before reclassifications	(142.8)	38.0	58.6
Amounts reclassified from accumulated other comprehensive loss	418.5	150.1	616.8
Current period other comprehensive income	275.7	188.1	675.4
Ending Balance	(138.4)	21.8	(64.4)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,076.0)	(3,264.1)	(3,939.5)
Ending Balance	(2,800.3)	(3,076.0)	(3,264.1)
Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(977.4)	(866.2)	(826.6)
Other comprehensive income (loss) before reclassifications	68.3	(114.1)	(43.6)
Amounts reclassified from accumulated other comprehensive loss	(3.7)	2.9	4.0
Current period other comprehensive income	64.6	(111.2)	(39.6)
Ending Balance	(912.8)	(977.4)	(866.2)
Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2,098.6)	(2,397.9)	(3,112.9)
Other comprehensive income (loss) before reclassifications	(211.1)	152.1	102.2
Amounts reclassified from accumulated other comprehensive loss	422.2	147.2	612.8
Current period other comprehensive income	211.1	299.3	715.0
Ending Balance	$ (1,887.5)	$ (2,098.6)	$ (2,397.9)