Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (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
Goodwill [Roll Forward]
Balance at beginning of period	[2]	$ 1,628.1	[1]	$ 1,722.2		$ 1,628.1	[1]	$ 1,722.2
Goodwill impairment charges		0.0		0.0	$ 0.0	(55.1)	[2],[3]	(50.0)	[2],[3]	$ 0.0
Acquisitions	[2]							2.3
Currency impact and other	[2]					13.8		(46.4)
Balance at end of period	[2]					1,586.8	[1]	1,628.1	[1]	1,722.2
Reportable segments | Americas
Goodwill [Roll Forward]
Balance at beginning of period	[4]	1,049.6		1,058.9		1,049.6		1,058.9
Goodwill impairment charges	[3],[4]					0.0		0.0
Acquisitions	[4]							(6.5)
Currency impact and other	[4]					0.8		(2.8)
Balance at end of period	[4]					1,050.4		1,049.6		1,058.9
Reportable segments | Southern Europe
Goodwill [Roll Forward]
Balance at beginning of period	[5]	148.2		146.7		148.2		146.7
Goodwill impairment charges	[3],[5]					0.0		0.0
Acquisitions	[5]							8.8
Currency impact and other	[5]					6.5		(7.3)
Balance at end of period	[5]					154.7		148.2		146.7
Reportable segments | Northern Europe
Goodwill [Roll Forward]
Balance at beginning of period		234.8		313.7		234.8		313.7
Goodwill impairment charges	[3]					(55.1)		(50.0)
Acquisitions								0.0
Currency impact and other						8.3		(28.9)
Balance at end of period						188.0		234.8		313.7
Reportable segments | APME
Goodwill [Roll Forward]
Balance at beginning of period		69.5		76.9		69.5		76.9
Goodwill impairment charges	[3]					0.0		0.0
Acquisitions								0.0
Currency impact and other						(1.8)		(7.4)
Balance at end of period						67.7		69.5		76.9
Corporate
Goodwill [Roll Forward]
Balance at beginning of period	[6]	$ 126.0		$ 126.0		126.0		126.0
Goodwill impairment charges	[3],[6]					0.0		0.0
Acquisitions								0.0
Currency impact and other						0.0		0.0
Balance at end of period	[6]					$ 126.0		$ 126.0		$ 126.0

[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] 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.

[5] 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.

[6] 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