Retirement Benefits - Change in Benefit Obligation, Change in Plan Assets, and Funded Status (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation:
Plan amendments	$ (21.0)
Pension Plans
Change in benefit obligation:
Benefit obligation at beginning of year		$ 234.8	$ 258.6
Service cost		9.3	11.3	$ 10.5
Interest cost		2.2	2.5	1.8
Actuarial loss (gain)		56.4	(44.7)
Benefits paid		(5.5)	(2.7)
Plan amendments		74.7	0.0
Foreign currency exchange rate changes and other adjustments		1.9	9.8
Benefit obligation at end of year		224.4	234.8	258.6
Change in plan assets:
Fair value of plan assets at beginning of year		131.6	131.5
Actual return on plan assets		15.3	(10.2)
Employer contribution		5.3	5.7
Benefits paid		(5.5)	(2.7)
Foreign currency exchange rate changes and other adjustments		2.0	7.3
Fair value of plan assets at end of year		148.7	131.6	$ 131.5
Funded status		(75.7)	(103.2)
Unrecognized net actuarial loss		45.9	0.5
Unrecognized prior service cost		(74.1)	0.8
Net amount recognized		$ (103.9)	$ (101.9)