Stockholders' equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (64.4)	$ (312.1)	$ (1,228.3)
Ending Balance	21.8	(64.4)	(312.1)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,264.1)	(3,939.5)	(4,088.6)
Other comprehensive income (loss) before reclassifications	38.0	58.6	489.4
Amounts reclassified from accumulated other comprehensive loss	150.1	616.8	(340.3)
Current period other comprehensive income (loss)	188.1	675.4	149.1
Ending Balance	(3,076.0)	(3,264.1)	(3,939.5)
Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(866.2)	(826.6)	(872.9)
Other comprehensive income (loss) before reclassifications	(114.1)	(43.6)	78.6
Amounts reclassified from accumulated other comprehensive loss	2.9	4.0	(32.3)
Current period other comprehensive income (loss)	(111.2)	(39.6)	46.3
Ending Balance	(977.4)	(866.2)	(826.6)
Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2,397.9)	(3,112.9)	(3,215.7)
Other comprehensive income (loss) before reclassifications	152.1	102.2	410.8
Amounts reclassified from accumulated other comprehensive loss	147.2	612.8	(308.0)
Current period other comprehensive income (loss)	299.3	715.0	102.8
Ending Balance	$ (2,098.6)	$ (2,397.9)	$ (3,112.9)