Goodwill and Other Intangible Assets - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Goodwill	$ 1,206.6	$ 1,225.8
Jefferson Wells
Goodwill [Roll Forward]
Goodwill	55.5
Right Management
Goodwill [Roll Forward]
Goodwill	62.1
Americas \| United States
Goodwill [Roll Forward]
Goodwill	493.3	490.2
Americas \| United States \| Jefferson Wells
Goodwill [Roll Forward]
Goodwill	572.1	545.7
Southern Europe \| France
Goodwill [Roll Forward]
Goodwill	75.6	73.3
Southern Europe \| Italy
Goodwill [Roll Forward]
Goodwill	$ 4.0	$ 4.2

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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
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-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
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-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant