Restructuring Costs - Schedule of Changes in Restructuring Reserve (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 7.3
Severance costs	37.0
Office closure and other costs	11.2
Costs paid, utilized or transferred out	(32.1)
Balance, end of period	23.4
Reportable segments | Americas
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.4
Severance costs	4.7
Office closure and other costs	8.1
Costs paid, utilized or transferred out	(10.5)
Balance, end of period	2.7
Reportable segments | Southern Europe
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.7
Severance costs	11.8
Office closure and other costs	1.3
Costs paid, utilized or transferred out	(12.0)
Balance, end of period	1.8
Reportable segments | Northern Europe
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	6.2
Severance costs	19.3
Office closure and other costs	0.2
Costs paid, utilized or transferred out	(7.0)
Balance, end of period	18.7
Reportable segments | APME
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.0
Severance costs	1.0
Office closure and other costs	1.6
Costs paid, utilized or transferred out	(2.6)
Balance, end of period	0.0
Corporate
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.0
Severance costs	0.2
Office closure and other costs	0.0
Costs paid, utilized or transferred out	0.0
Balance, end of period	$ 0.2