Share Plans - Share-based compensation expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	$ 29.1	$ 24.9	$ 29.8
Restricted Stock Units
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	15.0	14.6	13.0
Equity Option
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	4.3	3.7	3.4
Performance Shares
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	$ 9.8	$ 6.6	$ 13.4