Acquisitions Acxiom Acquisition (Details) - Business Acquisition, Acquiree [Domain] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Oct. 01, 2018
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Revenue	$ 10,230.4	$ 9,736.1
Finite-Lived Intangible Assets, Gross			$ 995.0
Cash and Cash Equivalents, at Carrying Value			13.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			2,327.9
Loans Payable	500.0
Debt Issuance Face Amount	2,000.0
Accounts receivable, net of allowance			112.9
Accounts receivable, billable to clients			8.3
Other current assets			28.2
Property, Plant and Equipment, Net			159.7
Deferred Income Taxes and Other Tax Liabilities, Noncurrent			(0.6)
Goodwill			1,110.8
Intangible Assets, Net (Excluding Goodwill)			995.0
Other non-current assets			8.3
Accounts Payable			(37.1)
Accrued Liabilities			(46.9)
Contract liabilities			(23.2)
Other non-current liabilities			(0.8)
Net revenue	181.7
Net Income (Loss) Attributable to Parent	20.4
Selling, General and Administrative Expense	35.0
Interest Expense	3.7
Interest Income	0.4
Other expense, net	10.3
Business Acquisition, Pro Forma Information, Description		27.6
Business Acquisition, Pro Forma Net Income (Loss)	$ 642.2	$ 519.2
Customer Lists [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross			600.0
Finite-Lived Intangible Asset, Useful Life	15 years
know-how technology [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross			235.0
Finite-Lived Intangible Asset, Useful Life	9 years
Trade Names [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross			$ 160.0