Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Parenthetical) (Details) - USD ($) $ in Millions		9 Months Ended			12 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021	Jan. 01, 2022
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill	[2]				$ 1,586.8	[1]	$ 1,628.1	[1]	$ 1,722.2
Goodwill, accumulated impairment loss					749.3		694.2
Impairment Charge		$ 0.0	$ 0.0	$ 0.0	55.1	[2],[3]	50.0	[2],[3]	0.0
Corporate
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill	[4]				126.0		126.0		126.0
Goodwill, accumulated impairment loss					278.2		278.2
Impairment Charge	[3],[4]				0.0		0.0
Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					55.5
Right Management
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill							62.1
Goodwill, accumulated impairment loss					235.2		235.2
Netherlands Subsegment [Member] | Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					0.0		55.1
Americas | Reportable segments
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill	[5]				1,050.4		1,049.6		1,058.9
Impairment Charge	[3],[5]				0.0		0.0
Americas | United States
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					1,006.5		1,006.5			$ 1,013.0
Americas | United States | Jefferson Wells
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					1,085.3		1,085.3
Southern Europe | Reportable segments
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill	[6]				154.7		148.2		146.7
Impairment Charge	[3],[6]				0.0		0.0
Southern Europe | France
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					81.7		79.5
Southern Europe | France | Right Management
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					75.6		73.3			68.2
Southern Europe | Italy
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					3.8		3.7			$ 3.9
Northern Europe
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill, accumulated impairment loss					232.1		177.0
Northern Europe | Reportable segments
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					188.0		234.8		313.7
Impairment Charge	[3]				55.1		50.0
Northern Europe | Netherlands Subsegment [Member]
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Impairment Charge					55.1
APME | Reportable segments
Goodwill, Impaired, Accumulated Impairment Loss [Abstract]
Goodwill					67.7		69.5		$ 76.9
Goodwill, accumulated impairment loss					3.8		3.8
Impairment Charge	[3]				$ 0.0		$ 0.0

[1] Balances were net of accumulated impairment loss of $749.3 and $694.2 as of December 31, 2023 and 2022, respectively.

[2] Balances were net of accumulated impairment loss of $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; and $749.3 ($232.1 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, 2023.

[3] The impairment charges relate to our Netherlands reporting unit, which was recorded during the fourth quarter of 2022 and 2023, respectively. See Note 1 to the Consolidated Financial Statements for further information.

[4] The majority of the Corporate balance as of December 31, 2022 and 2023 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

[5] Balances related to United States were $1,013.0, $1,006.5 and $1,006.5 as of January 1, 2022, December 31, 2022 and December 31, 2023, respectively. The 2022 reduction for acquisitions represents post-closing opening balance adjustments related to the Experis acquisition.

[6] Balances related to France were $68.2, $73.3 and $75.6 as of January 1, 2022, December 31, 2022 and December 31, 2023, respectively. Balances related to Italy were $3.9, $3.7 and $3.8 as of January 1, 2022, December 31, 2022 and December 31, 2023, respectively.