Income taxes - Component of Income Taxes Expenses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Currently payable:
U.S. federal		$ 7.1	$ 13.3	$ 12.3
State		(0.1)	0.9	0.3
Non-U.S.		14.4	25.3	13.5
Total		21.4	39.5	26.1
Deferred income taxes (benefit):
U.S. federal		(22.0)	38.6	(13.6)
State		(1.8)	2.7	(1.2)
Non-U.S.		14.3	2.1	(35.9)
Total	$ 16.5	(9.5)	43.4	(50.7)
Income tax expense		$ 11.9	$ 82.9	$ (24.6)