Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total revenues:
Total revenue	$ 2,025.7	$ 2,520.2	$ 4,385.5	$ 4,881.4
Net revenue	1,853.4	2,125.9	3,825.5	4,130.7
United States
Total revenues:
Total revenue	1,309.6	1,595.1	2,879.2	3,130.2
Net revenue	1,227.2	1,337.7	2,547.2	2,651.8
United Kingdom
Total revenues:
Total revenue	159.3	209.8	356.4	416.0
Net revenue	147.2	180.4	312.9	350.7
Continental Europe
Total revenues:
Total revenue	164.2	209.4	334.0	388.2
Net revenue	149.7	183.3	295.7	340.1
Asia Pacific
Total revenues:
Total revenue	200.4	258.2	411.8	490.6
Net revenue	162.6	205.1	321.4	383.1
Latin America
Total revenues:
Total revenue	65.7	101.8	152.5	191.1
Net revenue	62.3	92.1	141.6	172.4
Other
Total revenues:
Total revenue	126.5	145.9	251.6	265.3
Net revenue	104.4	127.3	206.7	232.6
International
Total revenues:
Total revenue	716.1	925.1	1,506.3	1,751.2
Net revenue	626.2	788.2	1,278.3	1,478.9
IAN
Total revenues:
Total revenue	1,672.5	1,959.6	3,491.3	3,821.8
Net revenue	1,585.7	1,801.1	3,250.2	3,507.2
IAN | United States
Total revenues:
Total revenue	1,078.9	1,206.8	2,272.2	2,406.9
Net revenue	1,048.4	1,127.0	2,160.3	2,241.2
IAN | International
Total revenues:
Total revenue	593.6	752.8	1,219.1	1,414.9
Net revenue	537.3	674.1	1,089.9	1,266.0
CMG
Total revenues:
Total revenue	353.2	560.6	894.2	1,059.6
Net revenue	267.7	324.8	575.3	623.5
CMG | United States
Total revenues:
Total revenue	230.7	388.3	607.0	723.3
Net revenue	178.8	210.7	386.9	410.6
CMG | International
Total revenues:
Total revenue	122.5	172.3	287.2	336.3
Net revenue	$ 88.9	$ 114.1	$ 188.4	$ 212.9