Retirement Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Fair value of plan assets at beginning of year	$ 743.4
Fair value of plan assets at end of year	805.1	$ 743.4
Pension Benefits
Change in benefit obligation
Benefit obligation at beginning of year	938.7	844.3
Service cost	1.2	2.2	$ 3.8
Interest cost	28.0	34.6	34.3
Plan participants’ contributions	0.0	0.4
Actuarial loss/(gain)	89.6	107.0
Curtailment gain	(0.1)	0.0
Settlements	(24.7)	0.0
Currency impact	5.1	3.6
Other	0.0	0.1
Benefits paid	(43.6)	(53.5)
Benefit obligation at end of year	994.2	938.7	844.3
Change in plan assets
Fair value of plan assets at beginning of year	743.4	670.1
Actual return on plan assets	94.9	106.2
Employer contributions	29.5	16.4
Plan participants’ contributions	0.0	0.4
Settlements	(24.7)	0.0
Currency impact	5.6	3.8
Benefits paid	(43.6)	(53.5)
Fair value of plan assets at end of year	805.1	743.4	670.1
NET AMOUNT RECOGNIZED IN THE CONSOLIDATED BALANCE SHEET	(189.1)	(195.3)
Prepaid pensions (included in Other long-term assets)	16.4	8.6
Accrued benefit liability (short-term and long-term)	(205.5)	(203.9)
NET AMOUNT RECOGNIZED IN THE CONSOLIDATED BALANCE SHEET	(189.1)	(195.3)
Amounts recognized in Accumulated other comprehensive loss (income) consist of:
Net actuarial loss	275.4	263.4
Prior service cost (credit)	3.5	3.6
NET AMOUNT RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE LOSS	278.9	267.0
Other Benefits
Change in benefit obligation
Benefit obligation at beginning of year	23.8	26.1
Service cost	0.0	0.1	0.1
Interest cost	0.7	1.1	1.0
Plan participants’ contributions	0.0	0.0
Actuarial loss/(gain)	0.2	(2.0)
Curtailment gain	0.0	0.0
Settlements	0.0	0.0
Currency impact	0.0	0.0
Other	0.0	0.0
Benefits paid	(1.5)	(1.5)
Benefit obligation at end of year	23.2	23.8	26.1
Change in plan assets
Fair value of plan assets at beginning of year	0.0	0.0
Actual return on plan assets	0.0	0.0
Employer contributions	1.5	1.5
Plan participants’ contributions	0.0	0.0
Settlements	0.0	0.0
Currency impact	0.0	0.0
Benefits paid	(1.5)	(1.5)
Fair value of plan assets at end of year	0.0	0.0	$ 0.0
NET AMOUNT RECOGNIZED IN THE CONSOLIDATED BALANCE SHEET	(23.2)	(23.8)
Prepaid pensions (included in Other long-term assets)	0.0	0.0
Accrued benefit liability (short-term and long-term)	(23.2)	(23.8)
NET AMOUNT RECOGNIZED IN THE CONSOLIDATED BALANCE SHEET	(23.2)	(23.8)
Amounts recognized in Accumulated other comprehensive loss (income) consist of:
Net actuarial loss	0.6	0.3
Prior service cost (credit)	0.0	(0.4)
NET AMOUNT RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE LOSS	$ 0.6	$ (0.1)