Revenue Recognition - Disaggregation of Revenue (Parenthetical) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	$ 4,856.1	$ 5,074.1	$ 9,608.4	$ 10,217.4
Outcome-Based Solutions and Consulting | United States
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				113.2		$ 60.4
Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax				65.2
Northern Europe
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		952.5	1,027.1	1,920.1	2,121.6
Northern Europe | Outcome-Based Solutions and Consulting
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		82.9	78.9	172.3	172.9
Northern Europe | Staffing and Interim
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 804.8	$ 874.0	$ 1,615.4	$ 1,812.1	$ 26.2

[1] Our consolidated revenues from services include fees received from our franchise offices of $3.3 and $3.9 for the three months ended June 30, 2023 and 2022, respectively, and $7.2 and $7.5 for the six months ended June 30, 2023 and 2022, respectively. These fees are primarily based on revenues generated by the franchise offices, which were $250.6 and $275.6 for the three months ended June 30, 2023 and 2022, respectively, and $500.8 and $540.8 for the six months ended June 30, 2023 and 2022, respectively.