Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) - USD ($)		3 Months Ended			12 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020
Goodwill [Roll Forward]
Balance at beginning of period	[2]				$ 1,722,200,000	[1]	$ 1,225,800,000
Goodwill Impairment charge		$ 0	$ 0	$ 0	(50,000,000.0)	[2],[3]	0		$ (66,800,000)
Acquisitions					2,300,000	[2]	522,700,000	[2]	0
Currency impact and other	[2]				(46,400,000)		(26,300,000)
Balance at end of period	[2]				1,628,100,000	[1]	1,722,200,000	[1]	1,225,800,000
Reportable segments | Americas
Goodwill [Roll Forward]
Balance at beginning of period	[4]				1,058,900,000		536,600,000
Goodwill Impairment charge
Acquisitions	[4]				(6,500,000)		522,700,000
Currency impact and other	[4]				(2,800,000)		(400,000)
Balance at end of period	[4]				1,049,600,000		1,058,900,000		536,600,000
Reportable segments | Southern Europe
Goodwill [Roll Forward]
Balance at beginning of period	[5]				146,700,000		154,900,000
Goodwill Impairment charge
Acquisitions					8,800,000	[5]
Currency impact and other	[5]				(7,300,000)		(8,200,000)
Balance at end of period	[5]				148,200,000		146,700,000		154,900,000
Reportable segments | Northern Europe
Goodwill [Roll Forward]
Balance at beginning of period					313,700,000		326,600,000
Goodwill Impairment charge	[3]				(50,000,000.0)
Acquisitions
Currency impact and other					(28,900,000)		(12,900,000)
Balance at end of period					234,800,000		313,700,000		326,600,000
Reportable segments | APME
Goodwill [Roll Forward]
Balance at beginning of period					76,900,000		81,700,000
Goodwill Impairment charge
Acquisitions
Currency impact and other					(7,400,000)		(4,800,000)
Balance at end of period					69,500,000		76,900,000		81,700,000
Corporate
Goodwill [Roll Forward]
Balance at beginning of period	[6]				126,000,000.0		126,000,000.0
Goodwill Impairment charge
Acquisitions
Currency impact and other
Balance at end of period	[6]				$ 126,000,000.0		$ 126,000,000.0		$ 126,000,000.0

[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] Balances related to United States were $490.2, $1,013.0 and $1,006.5 as of January 1, 2021, December 31, 2021 and December 31, 2022, respectively. The increase in 2021 is related to the Experis acquisition. The 2022 reduction for acquisitions represents post-closing opening balance adjustments related to the Experis acquisition.

[5] Balances related to France were $73.3, $68.2 and $73.3 as of January 1, 2021, December 31, 2021 and December 31, 2022, respectively. Balances related to Italy were $4.2, $3.9 and $3.7 as of January 1, 2021, December 31, 2021 and December 31, 2022, respectively.

[6] 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