Share Plans - Share-based compensation expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	$ 29.8	$ 20.3	$ 21.4
Restricted Stock Units
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	13.0	12.5	11.2
Equity Option
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	3.4	3.0	4.5
Performance Shares
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	$ 13.4	$ 4.8	$ 5.7