Goodwill and Other Intangible Assets - Changes in Goodwill by Reportable Segment (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Goodwill [Roll Forward]
Balance, January 1, 2018	$ 1,343.0
Goodwill acquired	4.6
Currency and other impacts	(25.7)
Balance, June 30, 2018	1,321.9
Right Management
Goodwill [Roll Forward]
Balance, January 1, 2018	62.1
Balance, June 30, 2018	62.1
Reportable segments \| Americas
Goodwill [Roll Forward]
Balance, January 1, 2018	519.2
Goodwill acquired	4.6
Currency and other impacts	(2.2)
Balance, June 30, 2018	521.6
Reportable segments \| Americas \| United States \| Reportable subsegments
Goodwill [Roll Forward]
Balance, January 1, 2018	476.5
Balance, June 30, 2018	476.5
Reportable segments \| Southern Europe
Goodwill [Roll Forward]
Balance, January 1, 2018	121.9
Goodwill acquired	0.0
Currency and other impacts	(7.5)
Balance, June 30, 2018	114.4
Reportable segments \| Southern Europe \| France \| Reportable subsegments
Goodwill [Roll Forward]
Balance, January 1, 2018	76.3
Balance, June 30, 2018	70.2
Reportable segments \| Southern Europe \| Italy \| Reportable subsegments
Goodwill [Roll Forward]
Balance, January 1, 2018	5.0
Balance, June 30, 2018	4.8
Reportable segments \| Northern Europe
Goodwill [Roll Forward]
Balance, January 1, 2018	468.1
Goodwill acquired	0.0
Currency and other impacts	(12.7)
Balance, June 30, 2018	455.4
Reportable segments \| APME
Goodwill [Roll Forward]
Balance, January 1, 2018	106.2
Goodwill acquired	0.0
Currency and other impacts	(3.3)
Balance, June 30, 2018	102.9
Reportable segments \| Right Management
Goodwill [Roll Forward]
Balance, January 1, 2018	62.1
Goodwill acquired	0.0
Currency and other impacts	0.0
Balance, June 30, 2018	62.1
Corporate
Goodwill [Roll Forward]
Balance, January 1, 2018	65.5
Goodwill acquired	0.0
Currency and other impacts	0.0
Balance, June 30, 2018	65.5
Corporate \| Jefferson Wells
Goodwill [Roll Forward]
Balance, June 30, 2018	$ 55.5

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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