Acquisitions Acxiom Acquisition (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	$ 1,330.0	$ 1,333.9
Cash and Cash Equivalents, at Carrying Value	3,270.0	2,509.0
Accounts receivable, net of allowance	5,177.7	4,646.4
Accounts receivable, billable to clients	2,347.2	1,820.7
Other current assets	428.7	390.7
Property, Plant and Equipment, Net	675.8	690.3
Goodwill	4,908.7	4,945.5	$ 4,894.4
Other non-current assets	399.6	424.4
Contract liabilities	(688.5)	(657.8)
Other non-current liabilities	(600.7)	(655.2)
Net revenue	9,107.9	8,064.5	8,625.1
Net Income Attributable to Parent	952.8	351.1	656.0
Goodwill and intangible assets related to acquisitions		28.0	$ 7.6
Customer Lists [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	855.7	859.5
know-how technology [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	235.3	235.4
Trade Names [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	$ 222.8	$ 224.4