Stockholders' equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (312.1)	$ (1,228.3)	$ (1,299.6)
Ending Balance	(64.4)	(312.1)	(1,228.3)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,939.5)	(4,088.6)	(4,084.8)
Other comprehensive income (loss) before reclassifications	58.6	489.4	136.8
Amounts reclassified from accumulated other comprehensive loss	616.8	(340.3)	(140.6)
Current period other comprehensive income (loss)	675.4	149.1	(3.8)
Ending Balance	(3,264.1)	(3,939.5)	(4,088.6)
Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(826.6)	(872.9)	(896.7)
Other comprehensive income (loss) before reclassifications	(43.6)	78.6	23.8
Amounts reclassified from accumulated other comprehensive loss	4.0	(32.3)
Current period other comprehensive income (loss)	(39.6)	46.3	23.8
Ending Balance	(866.2)	(826.6)	(872.9)
Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,112.9)	(3,215.7)	(3,188.1)
Other comprehensive income (loss) before reclassifications	102.2	410.8	113.0
Amounts reclassified from accumulated other comprehensive loss	612.8	(308.0)	(140.6)
Current period other comprehensive income (loss)	715.0	102.8	(27.6)
Ending Balance	$ (2,397.9)	$ (3,112.9)	$ (3,215.7)