Retirement Benefits - Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net asset value per share practical expedient(b)	$ 39.3	$ 37.2
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	355.8	215.1	$ 180.6
Non-U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	186.0	$ 177.9
Non-U.S. Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.3
Non-U.S. Plans | Level 3
Defined Benefit Plan Disclosure [Line Items]
Guaranteed contract value (Level 3)	$ 120.2