Retirement Benefits - Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net asset value per share practical expedient(a)	$ 17.2	$ 20.3
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	180.6	202.9	$ 173.4
Non-U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 163.4	$ 182.6