Summary of Significant Accounting Policies - Schedule of Changes in Restructuring Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	$ 15.5	$ 13.5
Severance costs	32.4	31.8
Office closure costs	9.6	7.5
Costs paid, utilized or transferred out	(50.2)	(37.3)
Balance at end of year	7.3	15.5
Reportable segments | Americas
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.3	1.7
Severance costs	3.6	0.3
Office closure costs	1.3	0.0
Costs paid, utilized or transferred out	(4.8)	(1.7)
Balance at end of year	0.4	0.3
Reportable segments | Southern Europe
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	1.7	0.9
Severance costs	5.2	5.4
Office closure costs	0.1	0.0
Costs paid, utilized or transferred out	(6.3)	(4.6)
Balance at end of year	0.7	1.7
Reportable segments | Northern Europe
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	13.1	9.6
Severance costs	17.6	25.8
Office closure costs	2.0	7.5
Costs paid, utilized or transferred out	(26.5)	(29.8)
Balance at end of year	6.2	13.1
Reportable segments | APME
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.0	0.0
Severance costs	3.5	0.0
Office closure costs	0.9	0.0
Costs paid, utilized or transferred out	(4.4)	0.0
Balance at end of year	0.0	0.0
Reportable segments | Right Management
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.4	1.2
Severance costs	0.2	0.3
Office closure costs	4.5	0.0
Costs paid, utilized or transferred out	(5.1)	(1.1)
Balance at end of year	0.0	0.4
Corporate
Restructuring Cost and Reserve [Line Items]
Balance, beginning of period	0.0	0.1
Severance costs	2.3	0.0
Office closure costs	0.8	0.0
Costs paid, utilized or transferred out	(3.1)	(0.1)
Balance at end of year	$ 0.0	$ 0.0