Accumulated other comprehensive income (loss) - Reclasses Out Of AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 605.0	$ 570.2	$ 369.8	$ 252.6	$ 570.2	$ 252.6
Other comprehensive income (loss)	30.3	36.0	(39.6)	49.4	66.3	9.8
Ending balance	664.2	605.0	366.7	369.8	664.2	366.7
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(290.7)			(290.7)
Other comprehensive income (loss) before reclassifications					(3.2)
Amounts reclassified from accumulated other comprehensive loss to net income					1.0
Other comprehensive income (loss)	(2.1)		9.1		(2.2)	16.1
Ending balance	(292.9)				(292.9)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(433.8)			(433.8)
Other comprehensive income (loss) before reclassifications					66.8
Amounts reclassified from accumulated other comprehensive loss to net income					(2.2)
Other comprehensive income (loss)	21.0		(52.5)		64.6	(18.9)
Ending balance	(369.2)				(369.2)
Unrealized gains (losses) on available-for-sale securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(0.6)			(0.6)
Other comprehensive income (loss) before reclassifications					0.3
Amounts reclassified from accumulated other comprehensive loss to net income					0.0
Other comprehensive income (loss)	1.5		(0.4)		0.3	(0.8)
Ending balance	(0.3)				(0.3)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		24.6			24.6
Other comprehensive income (loss) before reclassifications					8.2
Amounts reclassified from accumulated other comprehensive loss to net income					(1.8)
Other comprehensive income (loss)	12.9		9.2		6.4	19.9
Ending balance	31.0				31.0
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(664.7)	(700.5)	(857.0)	(907.9)	(700.5)	(907.9)
Other comprehensive income (loss) before reclassifications					72.1
Amounts reclassified from accumulated other comprehensive loss to net income					(3.0)
Other comprehensive income (loss)	33.3	35.8	(34.6)	50.9	69.1	16.3
Ending balance	$ (631.4)	$ (664.7)	$ (891.6)	$ (857.0)	$ (631.4)	$ (891.6)