Accumulated other comprehensive income (loss) - Reclasses Out Of AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 128.8
Other comprehensive loss	(42.8)	$ (117.4)
Ending balance	99.8
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(614.8)
Other comprehensive income (loss) before reclassifications	(13.2)
Amounts reclassified from accumulated other comprehensive loss to net income	11.5
Other comprehensive loss	(1.7)	14.5
Ending balance	(616.5)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(363.2)
Other comprehensive income (loss) before reclassifications	(44.2)
Amounts reclassified from accumulated other comprehensive loss to net income	0.0
Other comprehensive loss	(44.2)	(119.9)
Ending balance	(407.4)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(22.0)
Other comprehensive income (loss) before reclassifications	7.1
Amounts reclassified from accumulated other comprehensive loss to net income	(1.9)
Other comprehensive loss	5.2	(11.6)
Ending balance	(16.8)
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(1,000.0)
Other comprehensive income (loss) before reclassifications	(50.3)
Amounts reclassified from accumulated other comprehensive loss to net income	9.6
Other comprehensive loss	(40.7)	$ (117.0)
Ending balance	$ (1,040.7)