Acquisitions Acxiom Acquisition (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2023	Jan. 01, 2022
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	$ 1,365.1	$ 1,360.3
Cash and Cash Equivalents, at Carrying Value	2,386.1	2,545.3
Accounts receivable, net of allowance	5,768.8	5,316.0
Accounts receivable, billable to clients	2,229.2	2,023.0
Other current assets	128.6	83.7
Property, Plant and Equipment, Net	636.7	637.4
Goodwill	5,080.9	5,050.6	$ 4,908.7	$ 5,050.6	$ 4,908.7
Other non-current assets	428.1	431.2
Contract liabilities	(684.7)	(680.0)
Other non-current liabilities	(532.4)	(572.6)
Net revenue	9,400.6	9,449.4	9,107.9
Net Income Attributable to Parent	1,098.4	938.0	$ 952.8
Customer Lists [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	882.1	873.1
know-how technology [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	239.5	239.2
Trade Names [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	$ 227.3	$ 231.8