Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Total revenues:
Total revenue	$ 2,438.1	$ 2,297.5	$ 7,319.5	$ 6,858.4
Net revenue	2,061.4	1,895.7	6,192.1	5,617.9
United States
Total revenues:
Total revenue	1,550.9	1,407.1	4,681.1	4,203.5
Net revenue	1,313.0	1,160.9	3,964.8	3,424.7
United Kingdom
Total revenues:
Total revenue	201.9	220.9	617.9	626.9
Net revenue	172.1	174.0	522.8	513.2
Continental Europe
Total revenues:
Total revenue	177.6	177.5	565.8	564.3
Net revenue	155.7	152.0	495.8	489.4
Asia Pacific
Total revenues:
Total revenue	262.7	267.3	753.3	803.0
Net revenue	205.0	210.6	588.1	603.6
Latin America
Total revenues:
Total revenue	109.2	95.3	300.3	267.8
Net revenue	97.8	84.0	270.2	239.9
Other
Total revenues:
Total revenue	135.8	129.4	401.1	392.9
Net revenue	117.8	114.2	350.4	347.1
International
Total revenues:
Total revenue	887.2	890.4	2,638.4	2,654.9
Net revenue	748.4	734.8	2,227.3	2,193.2
IAN
Total revenues:
Total revenue	1,903.9	1,774.7	5,744.7	5,272.7
Net revenue	1,743.9	1,580.5	5,260.9	4,690.9
IAN | United States
Total revenues:
Total revenue	1,185.3	1,064.2	3,611.2	3,150.6
Net revenue	1,103.0	952.9	3,354.0	2,814.2
IAN | International
Total revenues:
Total revenue	718.6	710.5	2,133.5	2,122.1
Net revenue	640.9	627.6	1,906.9	1,876.7
CMG
Total revenues:
Total revenue	534.2	522.8	1,574.8	1,585.7
Net revenue	317.5	315.2	931.2	927.0
CMG | United States
Total revenues:
Total revenue	365.6	342.9	1,069.9	1,052.9
Net revenue	210.0	208.0	610.8	610.5
CMG | International
Total revenues:
Total revenue	168.6	179.9	504.9	532.8
Net revenue	$ 107.5	$ 107.2	$ 320.4	$ 316.5