Segment Data - Schedule of Segment Data (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
Segment Reporting Information [Line Items]
Revenues from services	[1]				$ 18,914.5		$ 19,827.5		$ 20,724.4
Corporate expenses					(153.7)		(157.0)		(154.3)
Goodwill impairment charges		$ 0.0	$ 0.0	$ 0.0	(55.1)	[2],[3]	(50.0)	[2],[3]	0.0
Intangible asset amortization expense	[4]				(34.6)		(37.1)		(24.2)
Operating profit					255.8		581.7		585.4
Interest and other expenses, net					(49.9)		(24.6)		(17.3)
Earnings before income taxes					205.9		557.1		568.1
Depreciation and amortization expense					88.6		84.6		73.4
Total assets					8,830.2		9,130.4		9,828.9
Additions to long-lived assets					588.6		525.6		528.4
Reportable segments
Segment Reporting Information [Line Items]
Depreciation and amortization expense					2.8		1.6		0.7
Corporate
Segment Reporting Information [Line Items]
Goodwill impairment charges	[3],[5]				0.0		0.0
Total assets	[6]				495.6		444.1		396.6
Additions to long-lived assets					10.7		9.0		14.0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Depreciation and amortization expense	[7]				34.6		37.1		24.2
Intercompany Eliminations
Segment Reporting Information [Line Items]
Revenues from services					(85.9)		(80.1)		(37.7)
Americas
Segment Reporting Information [Line Items]
Operating unit profit (loss)					165.6		282.6		195.2
Depreciation and amortization expense					11.7		10.0		9.3
Total assets					3,488.2		3,640.6		3,831.6
Additions to long-lived assets					104.4		93.8		93.2
Americas | Reportable segments
Segment Reporting Information [Line Items]
Goodwill impairment charges	[3],[8]				0.0		0.0
Southern Europe
Segment Reporting Information [Line Items]
Operating unit profit (loss)					357.7		413.0		416.3
Depreciation and amortization expense					19.3		17.7		20.0
Total assets					3,541.0		3,558.4		3,691.2
Additions to long-lived assets					289.4		253.5		237.3
Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Goodwill impairment charges	[3],[9]				0.0		0.0
Northern Europe
Segment Reporting Information [Line Items]
Operating unit profit (loss)					116.7		42.4		67.8
Northern Europe | Reportable segments
Segment Reporting Information [Line Items]
Goodwill impairment charges	[3]				(55.1)		(50.0)
Depreciation and amortization expense					13.9		10.7		10.9
Total assets					586.3		742.9		1,095.7
Northern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to long-lived assets					138.0		116.5		106.6
APME
Segment Reporting Information [Line Items]
Operating unit profit (loss)					92.6		87.8		84.6
APME | Reportable segments
Segment Reporting Information [Line Items]
Goodwill impairment charges	[3]				0.0		0.0
Depreciation and amortization expense					6.3		7.5		8.3
Total assets					719.1		744.4		813.8
Additions to long-lived assets					46.1		52.8		77.3
Northern Europe And APME
Segment Reporting Information [Line Items]
Operating unit profit (loss)					499.2		825.8		763.9
United States | Americas
Segment Reporting Information [Line Items]
Revenues from services					2,961.6		3,499.3
United States | Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and amortization expense					9.5		7.8		6.9
Total assets					3,054.8		3,243.2		3,434.6
United States | Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to long-lived assets					87.9		79.2		76.9
Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from services					1,453.2		1,436.4
Other Americas | Americas | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and amortization expense					2.2		2.2		2.4
Total assets					433.4		397.4		397.0
Other Americas | Americas | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to long-lived assets					16.5		14.6		16.3
France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					4,867.1		4,785.0
France | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and amortization expense					12.7		10.5		12.5
Total assets					2,253.6		2,281.9		2,353.3
Additions to long-lived assets					182.9		160.7		135.0
Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					1,708.8		1,706.9
Italy | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Depreciation and amortization expense					2.2		2.4		2.5
Total assets					487.0		455.3		509.7
Additions to long-lived assets					50.6		32.7		33.6
Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					1,939.4		2,044.4
Earnings from equity investments					4.4		4.8		5.0
Other Southern Europe | Southern Europe | Reportable segments
Segment Reporting Information [Line Items]
Total assets					800.4		821.2		828.2
Other Southern Europe | Southern Europe | Segment Reconciling Items
Segment Reporting Information [Line Items]
Additions to long-lived assets					55.9		60.1		68.7
Reportable subsegments
Segment Reporting Information [Line Items]
Revenues from services					19,000.4		19,907.6
Reportable subsegments | Americas
Segment Reporting Information [Line Items]
Revenues from services					4,414.8		4,935.7		4,263.7
Reportable subsegments | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					8,515.3		8,536.3		9,346.8
Reportable subsegments | Northern Europe
Segment Reporting Information [Line Items]
Revenues from services					3,748.0		4,048.3		4,670.5
Reportable subsegments | APME
Segment Reporting Information [Line Items]
Revenues from services					2,322.3		2,387.3		2,481.1
Reportable subsegments | Northern Europe And APME
Segment Reporting Information [Line Items]
Revenues from services					19,000.4		19,907.6		20,762.1
Reportable subsegments | United States | Americas
Segment Reporting Information [Line Items]
Revenues from services	[1]				2,961.6		3,499.3		2,743.3
Operating unit profit (loss)					100.4		219.2		136.0
Reportable subsegments | Other Americas | Americas
Segment Reporting Information [Line Items]
Revenues from services					1,453.2		1,436.4		1,520.4
Operating unit profit (loss)					65.2		63.4		59.2
Reportable subsegments | France | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					4,867.1		4,785.0		5,171.3
Operating unit profit (loss)					188.3		226.7		233.5
Reportable subsegments | Italy | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					1,708.8		1,706.9		1,795.4
Operating unit profit (loss)					124.7		122.9		115.3
Reportable subsegments | Other Southern Europe | Southern Europe
Segment Reporting Information [Line Items]
Revenues from services					1,939.4		2,044.4		2,380.1
Operating unit profit (loss)					$ 44.7		$ 63.4		$ 67.5

[1] (a) The United States revenues above represent revenues from our company-owned branches and franchise fees received from our franchise operations, which were $11.9, $12.8 and $12.8 for 2023, 2022, and 2021, 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] (b) Intangible asset amortization related to acquisitions is excluded from operating costs within the reportable segments and corporate expenses, and shown separately.

[5] 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

[6] (a) Corporate assets include assets that were not used in the operations of any segment, the most significant of which were purchased intangibles and cash.

[7] (a) Intangible asset amortization related to acquisitions is excluded from operating costs within the reportable segments and corporate expenses, and shown separately.

[8] 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.

[9] 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.