Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Total revenues:
Total revenue	$ 2,520.2	$ 2,391.8	$ 4,881.4	$ 4,560.9
Net revenue	2,125.9	1,948.2	4,130.7	3,722.2
United States
Total revenues:
Total revenue	1,595.1	1,445.7	3,130.2	2,796.4
Net revenue	1,337.7	1,171.5	2,651.8	2,263.8
United Kingdom
Total revenues:
Total revenue	209.8	201.6	416.0	406.0
Net revenue	180.4	175.7	350.7	339.2
Continental Europe
Total revenues:
Total revenue	209.4	205.1	388.2	386.8
Net revenue	183.3	178.7	340.1	337.4
Asia Pacific
Total revenues:
Total revenue	258.2	304.2	490.6	535.7
Net revenue	205.1	214.2	383.1	393.0
Latin America
Total revenues:
Total revenue	101.8	92.5	191.1	172.5
Net revenue	92.1	82.0	172.4	155.9
Other
Total revenues:
Total revenue	145.9	142.7	265.3	263.5
Net revenue	127.3	126.1	232.6	232.9
International
Total revenues:
Total revenue	925.1	946.1	1,751.2	1,764.5
Net revenue	788.2	776.7	1,478.9	1,458.4
IAN
Total revenues:
Total revenue	1,968.6	1,812.5	3,840.8	3,498.0
Net revenue	1,805.8	1,629.1	3,517.0	3,110.4
IAN | United States
Total revenues:
Total revenue	1,215.8	1,063.2	2,425.9	2,086.4
Net revenue	1,131.7	963.3	2,251.0	1,861.3
IAN | International
Total revenues:
Total revenue	752.8	749.3	1,414.9	1,411.6
Net revenue	674.1	665.8	1,266.0	1,249.1
CMG
Total revenues:
Total revenue	551.6	579.3	1,040.6	1,062.9
Net revenue	320.1	319.1	613.7	611.8
CMG | United States
Total revenues:
Total revenue	379.3	382.5	704.3	710.0
Net revenue	206.0	208.2	400.8	402.5
CMG | International
Total revenues:
Total revenue	172.3	196.8	336.3	352.9
Net revenue	$ 114.1	$ 110.9	$ 212.9	$ 209.3