Retirement Benefit Plans - Accumulated Other Comprehensive Loss (Details) - Accrued pension cost - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Net prior service credit	$ 2.7	$ 4.8
Accumulated other comprehensive loss	2.7	4.8	$ 6.4
International Plans
Defined Benefit Plan Disclosure [Line Items]
Net prior service credit	3.9	3.9
Accumulated other comprehensive loss	$ 3.9	$ 3.9	$ 4.0