Goodwill - Changes in Goodwill by Reportable Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Balance at beginning of period	$ 1,343.0	$ 1,239.9
Goodwill acquired	6.1	35.0
Goodwill allocated to business units sold	(8.8)
Currency impact and other	(43.2)	68.1
Balance at end of period	1,297.1	1,343.0
Accumulated impairment loss	513.4	513.4	$ 513.4
Americas | United States
Goodwill [Roll Forward]
Balance at beginning of period	476.5	476.5
Balance at end of period	476.5	476.5
Southern Europe | France
Goodwill [Roll Forward]
Balance at beginning of period	76.3	66.8
Balance at end of period	68.9	76.3
Southern Europe | Italy
Goodwill [Roll Forward]
Balance at beginning of period	5.0	4.4
Balance at end of period	4.8	5.0
Reportable segments | Americas
Goodwill [Roll Forward]
Balance at beginning of period	519.2	516.4
Goodwill acquired	4.6	0.0
Goodwill allocated to business units sold	0.0
Currency impact and other	(3.9)	2.8
Balance at end of period	519.9	519.2
Reportable segments | Southern Europe
Goodwill [Roll Forward]
Balance at beginning of period	121.9	97.0
Goodwill acquired	0.0	10.0
Goodwill allocated to business units sold	0.0
Currency impact and other	(9.7)	14.9
Balance at end of period	112.2	121.9
Reportable segments | Northern Europe
Goodwill [Roll Forward]
Balance at beginning of period	468.1	421.9
Goodwill acquired	0.0	0.1
Goodwill allocated to business units sold	(8.8)
Currency impact and other	(23.9)	46.1
Balance at end of period	435.4	468.1
Reportable segments | APME
Goodwill [Roll Forward]
Balance at beginning of period	106.2	77.0
Goodwill acquired	1.5	24.9
Goodwill allocated to business units sold	0.0
Currency impact and other	(5.7)	4.3
Balance at end of period	102.0	106.2
Reportable segments | Right Management
Goodwill [Roll Forward]
Balance at beginning of period	62.1	62.1
Goodwill acquired	0.0	0.0
Goodwill allocated to business units sold	0.0
Currency impact and other	0.0	0.0
Balance at end of period	62.1	62.1
Corporate
Goodwill [Roll Forward]
Balance at beginning of period	65.5	65.5
Goodwill acquired	0.0	0.0
Goodwill allocated to business units sold	0.0
Currency impact and other	0.0	0.0
Balance at end of period	65.5	$ 65.5
Corporate | Jefferson Wells
Goodwill [Roll Forward]
Balance at end of period	$ 55.5