Pension Plans - Reconciliations of Plan Benefit Obligation, Fair Value of Plan Assets and Funded Status of Pension Plans (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
Dec. 31, 2019	Oct. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1			$ 406.3	$ 362.0
Service cost			0.0	0.1	$ 0.1
Interest cost			8.4	11.1	10.4
Curtailment gain			0.0	(1.8)
Settlement gain	$ (2.7)		0.0	(2.7)
Actuarial loss			44.4	43.4
Exchange difference			12.7	10.0
Benefits paid			(17.2)	(15.8)
Prior service cost			0.1	0.0
Benefit obligation at December 31	406.3		454.7	406.3	362.0
Change in plan assets
Fair value of plan assets at January 1			371.1	322.0
Actual return on assets			33.0	52.1
Exchange difference			11.2	7.6
Contributions from employer			5.8	7.9
Benefits paid			(17.2)	(15.8)
Settlement loss			0.0	(2.7)
Fair value of plan assets at December 31	371.1		403.9	371.1	322.0
Funded status
Benefit obligations in excess of the fair value of plan assets	(35.2)		(50.8)	(35.2)
U.K.
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1			359.2	315.2
Service cost			0.0	0.0	0.0
Interest cost			7.0	9.2	8.6
Curtailment gain			0.0	0.0
Settlement gain			0.0	0.0
Actuarial loss			39.7	38.4
Exchange difference			12.7	10.0
Benefits paid			(14.7)	(13.6)
Prior service cost		$ 2.2	0.1	0.0
Benefit obligation at December 31	359.2		404.0	359.2	315.2
Change in plan assets
Fair value of plan assets at January 1			328.7	283.4
Actual return on assets			27.9	45.4
Exchange difference			11.2	7.6
Contributions from employer			5.8	5.9
Benefits paid			(14.7)	(13.6)
Settlement loss			0.0	0.0
Fair value of plan assets at December 31	328.7		358.9	328.7	283.4
Funded status
Benefit obligations in excess of the fair value of plan assets	(30.5)		(45.1)	(30.5)
U.S./ other
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1			47.1	46.8
Service cost			0.0	0.1	0.1
Interest cost			1.4	1.9	1.8
Curtailment gain			0.0	(1.8)
Settlement gain			0.0	(2.7)
Actuarial loss			4.7	5.0
Exchange difference			0.0	0.0
Benefits paid			(2.5)	(2.2)
Prior service cost			0.0	0.0
Benefit obligation at December 31	47.1		50.7	47.1	46.8
Change in plan assets
Fair value of plan assets at January 1			42.4	38.6
Actual return on assets			5.1	6.7
Exchange difference			0.0	0.0
Contributions from employer			0.0	2.0
Benefits paid			(2.5)	(2.2)
Settlement loss			0.0	(2.7)
Fair value of plan assets at December 31	42.4		45.0	42.4	$ 38.6
Funded status
Benefit obligations in excess of the fair value of plan assets	$ (4.7)		$ (5.7)	$ (4.7)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

Name:	us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.