Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total revenues:
Total revenue	$ 2,125.5	$ 2,438.1	$ 6,511.0	$ 7,319.5
Net revenue	1,954.6	2,061.4	5,780.1	6,192.1
United States
Total revenues:
Total revenue	1,354.8	1,550.9	4,234.0	4,681.1
Net revenue	1,273.4	1,313.0	3,820.6	3,964.8
United Kingdom
Total revenues:
Total revenue	172.4	201.9	528.8	617.9
Net revenue	162.0	172.1	474.9	522.8
Continental Europe
Total revenues:
Total revenue	174.5	177.6	508.5	565.8
Net revenue	158.0	155.7	453.7	495.8
Asia Pacific
Total revenues:
Total revenue	198.1	262.7	609.9	753.3
Net revenue	166.0	205.0	487.4	588.1
Latin America
Total revenues:
Total revenue	83.2	109.2	235.7	300.3
Net revenue	78.5	97.8	220.1	270.2
Other
Total revenues:
Total revenue	142.5	135.8	394.1	401.1
Net revenue	116.7	117.8	323.4	350.4
International
Total revenues:
Total revenue	770.7	887.2	2,277.0	2,638.4
Net revenue	681.2	748.4	1,959.5	2,227.3
IAN
Total revenues:
Total revenue	1,783.6	1,896.3	5,274.9	5,718.1
Net revenue	1,685.5	1,739.2	4,935.7	5,246.4
IAN | United States
Total revenues:
Total revenue	1,130.5	1,177.8	3,402.7	3,584.7
Net revenue	1,093.8	1,098.3	3,254.1	3,339.5
IAN | International
Total revenues:
Total revenue	653.1	718.5	1,872.2	2,133.4
Net revenue	591.7	640.9	1,681.6	1,906.9
CMG
Total revenues:
Total revenue	341.9	541.8	1,236.1	1,601.4
Net revenue	269.1	322.2	844.4	945.7
CMG | United States
Total revenues:
Total revenue	224.3	373.1	831.3	1,096.4
Net revenue	179.6	214.7	566.5	625.3
CMG | International
Total revenues:
Total revenue	117.6	168.7	404.8	505.0
Net revenue	$ 89.5	$ 107.5	$ 277.9	$ 320.4