Summary of Significant Accounting Policies - Schedule of Changes in Restructuring Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 7.3	$ 15.5
Severance costs	74.2	32.4
Office closure costs	27.3	9.6
Other costs	9.2
Costs paid, utilized or transferred out	(71.9)	(50.2)
Balance at end of year	46.1	7.3
Reportable segments | Americas
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.4	0.5
Severance costs	5.5	3.8
Office closure costs	19.6	5.8
Other costs	4.4
Costs paid, utilized or transferred out	(28.0)	(9.7)
Balance at end of year	1.9	0.4
Reportable segments | Southern Europe
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.7	1.9
Severance costs	16.2	5.2
Office closure costs	5.1	0.1
Other costs	3.2
Costs paid, utilized or transferred out	(21.7)	(6.5)
Balance at end of year	3.5	0.7
Reportable segments | Northern Europe
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	6.2	13.1
Severance costs	49.8	17.6
Office closure costs	2.1	2.0
Other costs	0.5
Costs paid, utilized or transferred out	(17.9)	(26.5)
Balance at end of year	40.7	6.2
Reportable segments | APME
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.0	0.0
Severance costs	2.5	3.5
Office closure costs	0.5	0.9
Other costs	1.1
Costs paid, utilized or transferred out	(4.1)	(4.4)
Balance at end of year	0.0	0.0
Corporate
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.0	0.0
Severance costs	0.2	2.3
Office closure costs	0.0	0.8
Other costs	0.0
Costs paid, utilized or transferred out	(0.2)	(3.1)
Balance at end of year	$ 0.0	$ 0.0