Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Total revenues:
Total revenue	$ 2,359.8	$ 2,361.2
Net revenue	1,972.1	2,004.8
United States
Total revenues:
Total revenue	1,569.6	1,535.1
Net revenue	1,320.0	1,314.1
United Kingdom
Total revenues:
Total revenue	197.1	206.2
Net revenue	165.7	170.3
Continental Europe
Total revenues:
Total revenue	169.8	178.8
Net revenue	146.0	156.8
Asia Pacific
Total revenues:
Total revenue	211.4	232.4
Net revenue	158.8	178.0
Latin America
Total revenues:
Total revenue	86.8	89.3
Net revenue	79.3	80.3
Other
Total revenues:
Total revenue	125.1	119.4
Net revenue	102.3	105.3
International
Total revenues:
Total revenue	790.2	826.1
Net revenue	652.1	690.7
IAN
Total revenues:
Total revenue	1,818.8	1,862.2
Net revenue	1,664.5	1,706.1
IAN | United States
Total revenues:
Total revenue	1,193.3	1,200.1
Net revenue	1,111.9	1,114.2
IAN | International
Total revenues:
Total revenue	625.5	662.1
Net revenue	552.6	591.9
CMG
Total revenues:
Total revenue	541.0	499.0
Net revenue	307.6	298.7
CMG | United States
Total revenues:
Total revenue	376.3	335.0
Net revenue	208.1	199.9
CMG | International
Total revenues:
Total revenue	164.7	164.0
Net revenue	$ 99.5	$ 98.8