Summary of Significant Accounting Policies - Additional Information (Details) $ / shares in Units, $ in Millions		1 Months Ended	9 Months Ended			12 Months Ended
		Jul. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2023 USD ($) Office Country $ / shares shares		Dec. 31, 2022 USD ($) $ / shares shares		Dec. 31, 2021 USD ($) $ / shares shares	Aug. 31, 2023 shares	Aug. 31, 2021 shares	Aug. 31, 2019 shares
Accounting Policies [Line Items]
Number of offices worldwide (over) | Office						2,100
Number of countries and territories | Country						75
Investments value						$ 102.2		$ 95.8
Unremitted earning from investments						$ 11.5		4.3
Payment terms						60 days
Description of timing						Our client contracts are generally short-term in nature with a term of one year or less. The timing between satisfaction of the performance obligation, invoicing and payment is not significant.
Deferred revenue						$ 31.9		35.6
Advertising expenses						28.9		29.6		$ 28.1
Restructuring costs		$ 82.5				149.2		3.6		15.2
Severance and employee related costs						53.3
Costs paid, utilized or transferred						72.4		13.7
Reserve will be paid by end of period						90.0		13.2		23.3
Consolidated amortization expense related to intangibles	[1]					34.6		37.1		24.2
2024						32.4
2025						30.6
2026						26.9
2027						26.3
2028						26.3
Goodwill, Impairment Loss			$ 0.0	$ 0.0	$ 0.0	$ 55.1	[2],[3]	50.0	[2],[3]	0.0
Discount rate for goodwill impairment test (as a percent)						50
Net capitalized software balance						$ 44.7		47.7
Amortization expense related to capitalized software cost						13.8		$ 9.8		$ 5.5
Asset Impairment Charges						$ 2.2
Property, Plant, and Equipment, Useful Life, Term, Description [Extensible Enumeration]						Property, Plant and Equipment, Net
Shares authorized to be repurchased (in shares) | shares											5,000,000	4,000,000	6,000,000
Shares repurchased (in shares) | shares						2,400,000		3,200,000
Total cost of shares repurchased						$ 179.8		$ 210.0
Shares repurchased, excluds excise tax						$ 1.7
Dividends declared (in dollars per share) | $ / shares						$ 2.94		$ 2.72		$ 2.52
Total dividend payments						$ 144.3		$ 139.9		$ 136.6
Net earnings, net of tax, attributable to noncontrolling interests						49.9		24.6		17.3
Cloud Computing Arrangements
Accounting Policies [Line Items]
Unamortized Implementation Costs						54.6		19.9
Noncontrolling Interests
Accounting Policies [Line Items]
Net earnings, net of tax, attributable to noncontrolling interests						$ 0.4		$ 0.8		$ 0.7
August 2019 Authorization
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares								1,200,000		2,100,000
Shares remaining authorized for repurchase (in shares) | shares						0
August 2019 Authorization | Common Stock
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares						2,000,000
August 2021 Authorization
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares						2,000,000
Shares remaining authorized for repurchase (in shares) | shares						0
August 2021 Authorization | Common Stock
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares						270,000,000
August 2023 Authorization
Accounting Policies [Line Items]
Shares repurchased (in shares) | shares						400,000
Shares remaining authorized for repurchase (in shares) | shares						4,600,000
Buildings
Accounting Policies [Line Items]
Estimated useful lives						40 years
Netherlands
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)						0.125
Netherlands | Discount Rates
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)						0.132
Netherlands | Terminal Value Revenue Growth Rate
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)			0.02			0.02
Netherlands | Terminal Value OUP Margin
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)			0.032			0.03
Northern Europe | Netherlands
Accounting Policies [Line Items]
Goodwill, Impairment Loss						$ 55.1
Minimum
Accounting Policies [Line Items]
Percentage of goodwill over fair value of reporting unit						20.00%
Remaining lease terms						1 month
Minimum | Furniture, Fixtures, and Autos
Accounting Policies [Line Items]
Estimated useful lives						3 years
Minimum | Computer Equipment
Accounting Policies [Line Items]
Estimated useful lives						3 years
Minimum | Netherlands
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)			(0.012)
Minimum | Netherlands | Revenue Growth
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)						(0.063)
Maximum
Accounting Policies [Line Items]
Remaining lease terms						15 years
Maximum | Furniture, Fixtures, and Autos
Accounting Policies [Line Items]
Estimated useful lives						16 years
Maximum | Computer Equipment
Accounting Policies [Line Items]
Estimated useful lives						16 years
Maximum | Netherlands
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)			0.064
Maximum | Netherlands | Revenue Growth
Accounting Policies [Line Items]
Discount rate for goodwill impairment test (as a percent)						0.03
Customer relationships
Accounting Policies [Line Items]
Useful life						14 years
Other
Accounting Policies [Line Items]
Useful life						4 years
Computer software | Minimum
Accounting Policies [Line Items]
Useful life						3 years
Computer software | Maximum
Accounting Policies [Line Items]
Useful life						10 years
Other Reporting Units | Minimum
Accounting Policies [Line Items]
Percentage of goodwill over fair value of reporting unit						20.00%
Euro-denominated notes | Estimate of fair value measurement | Significant Other Observable Inputs (Level 2)
Accounting Policies [Line Items]
Carrying value of long-term debt						$ 977.6		$ 921.7
Euro-denominated notes | Carrying value | Significant Other Observable Inputs (Level 2)
Accounting Policies [Line Items]
Carrying value of long-term debt						$ 988.2		$ 956.6

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

[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.