Credit Facilities (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Short-term Debt [Line Items]
Post Acxiom Acquisition Quarters 1-3 Maximum Leverage Ratio Covenant	4.00 to 1.00
Post Acxiom Acquisition Quarters 4-7 Maximum Leverage Ratio Covenant	3.75 to 1.00
Post Acxiom Acquisition Quarters 8-Thereafter Maximum Leverage Ratio Covenant	3.50 to 1.00
Committed credit facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Expiration Date		Oct. 25, 2022
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,500.0	$ 1,500.0
Line of Credit Facility, Available Capacity Increase Amount	250.0	250.0
Line of Credit Facility, Limit on Letters of Credit	50.0	50.0
Line of Credit Outstanding, Amount	0.0	0.0
Letters of Credit Outstanding, Amount	8.5	8.5
Line of Credit Facility, Remaining Borrowing Capacity	1,491.5	1,491.5
Uncommitted credit facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,157.0	1,157.0
Line of Credit Outstanding, Amount	$ 66.6	66.6
Line of Credit Facility, Average Outstanding Amount		$ 100.6
Short-term Debt, Weighted Average Interest Rate, over Time		4.30%