Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Total revenues:
Total revenue	$ 2,169.1	$ 2,063.8
Net revenue	1,774.0	1,675.3
United States
Total revenues:
Total revenue	1,350.7	1,315.2
Net revenue	1,092.3	1,057.1
Total International
Total revenues:
Total revenue	818.4	748.6
Net revenue	681.7	618.2
United Kingdom
Total revenues:
Total revenue	204.4	175.0
Net revenue	163.5	135.2
Continental Europe
Total revenues:
Total revenue	181.7	158.2
Net revenue	158.7	140.9
Asia Pacific
Total revenues:
Total revenue	231.5	220.3
Net revenue	178.8	173.7
Latin America
Total revenues:
Total revenue	80.0	75.8
Net revenue	73.9	69.0
Other
Total revenues:
Total revenue	120.8	119.3
Net revenue	106.8	99.4
IAN
Total revenues:
Total revenue	1,685.5	1,567.8
Net revenue	1,481.3	1,391.1
IAN | United States
Total revenues:
Total revenue	1,023.2	969.9
Net revenue	898.0	862.0
IAN | Total International
Total revenues:
Total revenue	662.3	597.9
Net revenue	583.3	529.1
CMG
Total revenues:
Total revenue	483.6	496.0
Net revenue	292.7	284.2
CMG | United States
Total revenues:
Total revenue	327.5	345.3
Net revenue	194.3	195.1
CMG | Total International
Total revenues:
Total revenue	156.1	150.7
Net revenue	$ 98.4	$ 89.1