Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($) $ in Millions		3 Months Ended					12 Months Ended
		Dec. 31, 2022		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020	Jan. 01, 2021
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill	[2]	$ 1,628.1	[1]				$ 1,628.1	[1]	$ 1,722.2	[1]	$ 1,225.8
Goodwill, accumulated impairment loss		694.2					694.2		644.2			$ 644.2
Impairment Charge				$ 0.0	$ 0.0	$ 0.0	50.0	[2],[3]	0.0		66.8
Corporate
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill	[4]	126.0					126.0		126.0		$ 126.0
Goodwill, accumulated impairment loss		278.2					278.2		278.2			278.2
Impairment Charge
Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		55.5					55.5
Right Management
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill									62.1
Goodwill, accumulated impairment loss		235.2					235.2		235.2			235.2
Netherlands Subsegment [Member]
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Impairment Charge							50.0
Netherlands Subsegment [Member] | Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		55.1					55.1		74.3
Americas | United States
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		1,006.5					1,006.5		1,013.0			490.2
Americas | United States | Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		1,085.3					1,085.3		1,091.7
Southern Europe | France
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		79.5					79.5		110.7
Southern Europe | France | Right Management
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		73.3					73.3		68.2			73.3
Southern Europe | Italy
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		3.7					3.7		3.9			4.2
Northern Europe
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill, accumulated impairment loss		177.0					177.0		127.0			127.0
Northern Europe | Netherlands Subsegment [Member]
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill		55.1					55.1
Impairment Charge		50.0
APME
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill, accumulated impairment loss		$ 3.8					$ 3.8		$ 3.8			$ 3.8

[1] Balances were net of accumulated impairment loss of $694.2 and $644.2 as of December 31, 2022 and 2021, respectively.

[2] Balances were net of accumulated impairment loss of $644.2 ($127.0 related to Northern Europe, $3.8 related to APME, $235.2 related to Right Management and $278.2 related to corporate) as of both January 1, 2021 and December 31, 2021; and $694.2 ($177.0 related to Northern Europe, $3.8 related to APME, $235.2 related to Right Management and $278.2 related to Corporate) as of December 31, 2022.

[3] The 2022 impairment charge of $50.0 relates to our Netherlands reporting unit, which was recorded during the fourth quarter of 2022. See Note 1 to the Consolidated Financial Statements for further information.

[4] The majority of the Corporate balance as of December 31, 2021 and 2022 relates to goodwill attributable to our acquisitions of Right Management ($62.1) and Jefferson Wells ($55.5). Jefferson Wells is part of the United States reporting unit. Right Management is allocated to the reporting units of the countries in which Right Management operates. For purposes of monitoring our total assets by segment, we do not allocate the Corporate balance to the respective reportable segments as this is commensurate with how we operate our business. We do, however, include these balances within the appropriate reporting units for our goodwill impairment testing. See table below for the breakout of goodwill balances by reporting unit