Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Total revenues:
Total revenue	$ 2,361.2	$ 2,169.1
Net revenue	2,004.8	1,774.0
Net revenue	2,004.8	1,774.0
United States
Total revenues:
Total revenue	1,535.1	1,350.7
Net revenue	1,314.1	1,092.3
International
Total revenues:
Total revenue	826.1	818.4
Net revenue	690.7	681.7
United Kingdom
Total revenues:
Total revenue	206.2	204.4
Net revenue	170.3	163.5
Continental Europe
Total revenues:
Total revenue	178.8	181.7
Net revenue	156.8	158.7
Asia Pacific
Total revenues:
Total revenue	232.4	231.5
Net revenue	178.0	178.8
Latin America
Total revenues:
Total revenue	89.3	80.0
Net revenue	80.3	73.9
Other
Total revenues:
Total revenue	119.4	120.8
Net revenue	105.3	106.8
IAN
Total revenues:
Total revenue	1,872.2	1,685.5
Net revenue	1,711.2	1,481.3
IAN | United States
Total revenues:
Total revenue	1,210.1	1,023.2
Net revenue	1,119.3	898.0
IAN | International
Total revenues:
Total revenue	662.1	662.3
Net revenue	591.9	583.3
CMG
Total revenues:
Total revenue	489.0	483.6
Net revenue	293.6	292.7
CMG | United States
Total revenues:
Total revenue	325.0	327.5
Net revenue	194.8	194.3
CMG | International
Total revenues:
Total revenue	164.0	156.1
Net revenue	$ 98.8	$ 98.4