Accumulated other comprehensive income (loss) - Reclasses Out Of AOCI (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 446.6	$ 519.9	$ 520.2	$ 664.2	$ 605.0	$ 570.2	$ 520.2	$ 570.2
Other comprehensive income (loss)	(2.7)	(75.9)	(14.8)	(30.7)	30.3	36.0	(93.4)	35.6
Ending balance	408.1	446.6	519.9	584.3	664.2	605.0	408.1	584.3
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(744.0)	(669.0)	(656.0)	(631.4)	(664.7)	(700.5)	(656.0)	(700.5)
Other comprehensive income (loss) before reclassifications							(83.5)
Amounts reclassified from accumulated other comprehensive loss to net income							(14.2)
Other comprehensive income (loss)	(9.7)	(75.0)	(13.0)	(29.2)	33.3	35.8	(97.7)	39.9
Ending balance	(753.7)	$ (744.0)	(669.0)	(660.6)	$ (631.4)	$ (664.7)	(753.7)	(660.6)
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(302.2)				(302.2)
Other comprehensive income (loss) before reclassifications							(7.5)
Amounts reclassified from accumulated other comprehensive loss to net income							3.8
Other comprehensive income (loss)	(0.4)			1.3			(3.7)	(0.9)
Ending balance	(305.9)						(305.9)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(368.2)				(368.2)
Other comprehensive income (loss) before reclassifications							(79.0)
Amounts reclassified from accumulated other comprehensive loss to net income							(2.6)
Other comprehensive income (loss)	11.6			(41.9)			(81.6)	22.7
Ending balance	(449.8)						(449.8)
Gains (losses) on available-for-sale securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			(1.8)				(1.8)
Other comprehensive income (loss) before reclassifications							(0.4)
Amounts reclassified from accumulated other comprehensive loss to net income							(4.5)
Other comprehensive income (loss)	(4.3)			5.1			(4.9)	5.4
Ending balance	(6.7)						(6.7)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance			$ 16.2				16.2
Other comprehensive income (loss) before reclassifications							3.4
Amounts reclassified from accumulated other comprehensive loss to net income							(10.9)
Other comprehensive income (loss)	(16.6)			$ 6.3			(7.5)	$ 12.7
Ending balance	$ 8.7						$ 8.7