Share Plans - Share-based compensation expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	$ 21.4	$ 20.9	$ 39.6
Restricted Stock Units
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	11.2	8.9	17.5
Equity Option
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	4.5	4.6	10.5
Performance Shares
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation	$ 5.7	$ 7.4	$ 11.6