Accumulated other comprehensive income (loss) - Reclasses out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2026	Mar. 31, 2026	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2026	Jun. 30, 2025
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 393.1	$ 407.3	$ 329.0	$ 312.5	$ 407.3	$ 312.5
Other comprehensive income	6.5	0.4	93.2	38.5	6.9	131.7
Ending balance	442.4	393.1	385.1	329.0	442.4	385.1
AOCI Attributable to Parent
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(664.2)	(665.6)	(766.5)	(804.1)	(665.6)	(804.1)
Other comprehensive income (loss) before reclassifications					5.4
Amounts reclassified from accumulated other comprehensive loss to net income					2.6
Other comprehensive income	6.6	1.4	89.3	37.8	8.0	127.1
Ending balance	(657.6)	(664.2)	(677.2)	$ (766.5)	(657.6)	(677.2)
Benefit Plan Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(267.0)			(267.0)
Other comprehensive income (loss) before reclassifications					(0.8)
Amounts reclassified from accumulated other comprehensive loss to net income					4.7
Other comprehensive income	2.1		(1.9)		3.9	(3.5)
Ending balance	(263.1)				(263.1)
Foreign Currency Translation Adjustments
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(406.8)			(406.8)
Other comprehensive income (loss) before reclassifications					5.9
Amounts reclassified from accumulated other comprehensive loss to net income					(2.1)
Other comprehensive income	5.0		91.9		3.8	132.6
Ending balance	(403.0)				(403.0)
Gains (losses) on available-for-sale securities
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		(0.9)			(0.9)
Other comprehensive income (loss) before reclassifications					(0.6)
Amounts reclassified from accumulated other comprehensive loss to net income					1.6
Other comprehensive income	(0.1)		1.1		1.0	1.8
Ending balance	0.1				0.1
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance		$ 9.1			9.1
Other comprehensive income (loss) before reclassifications					0.9
Amounts reclassified from accumulated other comprehensive loss to net income					(1.6)
Other comprehensive income	(0.4)		$ (1.8)		(0.7)	$ (3.8)
Ending balance	$ 8.4				$ 8.4