Pension Plans - Reconciliations of Plan Benefit Obligation, Fair Value of Plan Assets and Funded Status of Pension Plans (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	$ 422.8	$ 383.4
Service cost	0.1	0.1	$ 0.4
Interest cost	10.4	10.8	13.4
Actuarial (gain) / loss	(33.6)	14.4
Exchange difference	(19.8)	32.3
Benefits paid	(20.1)	(18.2)
Prior service cost	2.2	0.0
Benefit obligation at December 31	362.0	422.8	383.4
Change in plan assets
Fair value of plan assets at January 1	367.5	316.9
Actual return on assets	(15.5)	32.5
Exchange difference	(17.8)	27.4
Contributions from employer	7.9	8.9
Benefits paid	(20.1)	(18.2)
Fair value of plan assets at December 31	322.0	367.5	316.9
Funded status
Benefit obligations in excess of the fair value of plan assets	(40.0)	(55.3)
U.K.
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	369.7	334.8
Service cost	0.0	0.0	0.3
Interest cost	8.6	8.9	10.9
Actuarial (gain) / loss	(27.7)	10.1
Exchange difference	(19.7)	32.0
Benefits paid	(17.9)	(16.1)
Prior service cost	2.2	0.0
Benefit obligation at December 31	315.2	369.7	334.8
Change in plan assets
Fair value of plan assets at January 1	326.3	280.3
Actual return on assets	(13.0)	27.7
Exchange difference	(17.8)	27.4
Contributions from employer	5.8	7.0
Benefits paid	(17.9)	(16.1)
Fair value of plan assets at December 31	283.4	326.3	280.3
Funded status
Benefit obligations in excess of the fair value of plan assets	(31.8)	(43.4)
U.S./ other
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	53.1	48.6
Service cost	0.1	0.1	0.1
Interest cost	1.8	1.9	2.5
Actuarial (gain) / loss	(5.9)	4.3
Exchange difference	(0.1)	0.3
Benefits paid	(2.2)	(2.1)
Prior service cost	0.0	0.0
Benefit obligation at December 31	46.8	53.1	48.6
Change in plan assets
Fair value of plan assets at January 1	41.2	36.6
Actual return on assets	(2.5)	4.8
Exchange difference	0.0	0.0
Contributions from employer	2.1	1.9
Benefits paid	(2.2)	(2.1)
Fair value of plan assets at December 31	38.6	41.2	$ 36.6
Funded status
Benefit obligations in excess of the fair value of plan assets	$ (8.2)	$ (11.9)

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.