Accumulated other comprehensive income (loss) - Reclasses Out Of AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		$ 128.8			$ 128.8
Other comprehensive income (loss)	$ 49.5	(42.8)	$ 35.2	$ (117.4)	6.7	$ (82.2)
Ending balance	174.0				174.0
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(614.8)			(614.8)
Other comprehensive income (loss) before reclassifications					(13.3)
Amounts reclassified from accumulated other comprehensive loss to net income					22.9
Other comprehensive income (loss)	11.3		8.8		9.6	23.3
Ending balance	(605.2)				(605.2)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(363.2)			(363.2)
Other comprehensive income (loss) before reclassifications					(7.5)
Amounts reclassified from accumulated other comprehensive loss to net income					(0.9)
Other comprehensive income (loss)	35.8		27.2		(8.4)	(92.7)
Ending balance	(371.6)				(371.6)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(22.0)			(22.0)
Other comprehensive income (loss) before reclassifications					(0.6)
Amounts reclassified from accumulated other comprehensive loss to net income					7.9
Other comprehensive income (loss)	2.1		(1.8)		7.3	(13.4)
Ending balance	(14.7)				(14.7)
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(1,000.0)			(1,000.0)
Other comprehensive income (loss) before reclassifications					(21.4)
Amounts reclassified from accumulated other comprehensive loss to net income					29.9
Other comprehensive income (loss)	49.2	$ (40.7)	$ 34.2	$ (117.0)	8.5	$ (82.8)
Ending balance	$ (991.5)				$ (991.5)