Income taxes - Component of Income Taxes Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Currently payable:
U.S. federal and state	$ 12.6	$ 16.3	$ 29.7
Non-U.S.	13.5	20.1	21.5
Total	26.1	36.4	51.2
Deferred income taxes (benefit):
U.S. federal and state	(12.6)	(3.9)	(1.7)
Non-U.S.	(35.9)	1.3	10.6
Total	(48.5)	(2.6)	8.9
Income tax expense	$ (22.4)	$ 33.8	$ 60.1