Accumulated other comprehensive income (loss) - Reclasses Out Of AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 369.8	$ 252.6	$ 225.1	$ 202.5	$ 252.6	$ 202.5
Other comprehensive income (loss)	(39.6)	49.4	49.5	(42.8)	9.8	6.7
Ending balance	366.7	369.8	302.6	225.1	366.7	302.6
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(474.0)			(474.0)
Other comprehensive income (loss) before reclassifications					1.1
Amounts reclassified from accumulated other comprehensive loss to net income					15.0
Other comprehensive income (loss)	9.1		11.3		16.1	9.6
Ending balance	(457.9)				(457.9)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(425.7)			(425.7)
Other comprehensive income (loss) before reclassifications					(16.6)
Amounts reclassified from accumulated other comprehensive loss to net income					(2.3)
Other comprehensive income (loss)	(52.5)		35.8		(18.9)	(8.4)
Ending balance	(444.6)				(444.6)
Unrealized losses on available-for-sale securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(0.1)			(0.1)
Other comprehensive income (loss) before reclassifications					(0.8)
Amounts reclassified from accumulated other comprehensive loss to net income					0.0
Other comprehensive income (loss)	(0.4)				(0.8)
Ending balance	(0.9)				(0.9)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(8.1)			(8.1)
Other comprehensive income (loss) before reclassifications					12.0
Amounts reclassified from accumulated other comprehensive loss to net income					7.9
Other comprehensive income (loss)	9.2		2.1		19.9	7.3
Ending balance	11.8				11.8
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(857.0)	(907.9)	(1,040.7)	(1,000.0)	(907.9)	(1,000.0)
Other comprehensive income (loss) before reclassifications					(4.3)
Amounts reclassified from accumulated other comprehensive loss to net income					20.6
Other comprehensive income (loss)	(34.6)	50.9	49.2	(40.7)	16.3	8.5
Ending balance	$ (891.6)	$ (857.0)	$ (991.5)	$ (1,040.7)	$ (891.6)	$ (991.5)