Summary of Significant Accounting Policies - Restructuring Costs (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 39,300,000	$ 34,500,000
Restructuring reserve [Roll Forward]
Balance at beginning of period	13,500,000	4,500,000
Severance costs	31,800,000	24,700,000
Office closure costs	7,500,000	9,800,000
Costs paid or utilized	(37,300,000)	(25,500,000)
Balance at end of year	15,500,000	13,500,000
Reportable segments | Americas
Restructuring reserve [Roll Forward]
Balance at beginning of period	1,700,000	400,000
Severance costs	300,000	5,800,000
Office closure costs	0	500,000
Costs paid or utilized	(1,700,000)	(5,000,000)
Balance at end of year	300,000	1,700,000
Reportable segments | Americas | United States | Reportable subsegments
Restructuring reserve [Roll Forward]
Balance at beginning of period	1,500,000	400,000
Severance costs		3,700,000
Office closure costs		500,000
Costs paid or utilized	(1,200,000)	(3,100,000)
Balance at end of year	300,000	1,500,000
Reportable segments | Southern Europe
Restructuring reserve [Roll Forward]
Balance at beginning of period	900,000	1,300,000
Severance costs	5,400,000	0
Office closure costs	0	0
Costs paid or utilized	(4,600,000)	(400,000)
Balance at end of year	1,700,000	900,000
Reportable segments | Southern Europe | France | Reportable subsegments
Restructuring reserve [Roll Forward]
Balance at beginning of period	900,000	1,300,000
Costs paid or utilized		(400,000)
Balance at end of year	900,000	900,000
Reportable segments | Southern Europe | Italy | Reportable subsegments
Restructuring reserve [Roll Forward]
Balance at beginning of period	0	0
Severance costs	1,900,000
Costs paid or utilized	(1,400,000)
Balance at end of year	500,000	0
Reportable segments | Northern Europe
Restructuring reserve [Roll Forward]
Balance at beginning of period	9,600,000	2,600,000
Severance costs	25,800,000	15,600,000
Office closure costs	7,500,000	8,200,000
Costs paid or utilized	(29,800,000)	(16,800,000)
Balance at end of year	13,100,000	9,600,000
Reportable segments | APME
Restructuring reserve [Roll Forward]
Balance at beginning of period	0	100,000
Severance costs	0	900,000
Office closure costs	0	500,000
Costs paid or utilized	0	(1,500,000)
Balance at end of year	0	0
Reportable segments | Right Management
Restructuring reserve [Roll Forward]
Balance at beginning of period	1,200,000	100,000
Severance costs	300,000	1,400,000
Office closure costs	0	600,000
Costs paid or utilized	(1,100,000)	(900,000)
Balance at end of year	400,000	1,200,000
Corporate
Restructuring reserve [Roll Forward]
Balance at beginning of period	100,000	0
Severance costs	0	1,000,000
Office closure costs	0	0
Costs paid or utilized	(100,000)	(900,000)
Balance at end of year	$ 0	$ 100,000