Income taxes - Component of Income Taxes Expenses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Currently payable:
U.S. federal and state		$ 14.2	$ 12.6	$ 16.3
Non-U.S.		25.3	13.5	20.1
Total		39.5	26.1	36.4
Deferred income taxes (benefit):
U.S. federal and state		41.3	(14.8)	(0.9)
Non-U.S.		2.1	(35.9)	1.3
Total	$ 16.5	43.4	(50.7)	0.4
Income tax expense (benefit)		$ 82.9	$ (24.6)	$ 36.8