Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Total revenues:
Net Revenue	$ 1,948.2	$ 1,834.6	$ 3,722.2	$ 3,509.9
Total revenue	2,391.8	2,185.8	4,560.9	4,249.6
United States
Total revenues:
Net Revenue	1,171.5	1,127.6	2,263.8	2,184.7
Total revenue	1,445.7	1,350.8	2,796.4	2,666.0
Total International
Total revenues:
Net Revenue	776.7	707.0	1,458.4	1,325.2
Total revenue	946.1	835.0	1,764.5	1,583.6
United Kingdom
Total revenues:
Net Revenue	175.7	141.7	339.2	276.9
Total revenue	201.6	166.0	406.0	341.0
Continental Europe
Total revenues:
Net Revenue	178.7	154.6	337.4	295.5
Total revenue	205.1	176.3	386.8	334.5
Asia Pacific
Total revenues:
Net Revenue	214.2	205.3	393.0	379.0
Total revenue	304.2	260.5	535.7	480.8
Latin America
Total revenues:
Net Revenue	82.0	85.7	155.9	154.7
Total revenue	92.5	94.5	172.5	170.3
Other
Total revenues:
Net Revenue	126.1	119.7	232.9	219.1
Total revenue	142.7	137.7	263.5	257.0
IAN
Total revenues:
Net Revenue	1,629.1	1,534.0	3,110.4	2,925.1
Total revenue	1,812.5	1,711.3	3,498.0	3,279.1
IAN | United States
Total revenues:
Net Revenue	963.3	926.3	1,861.3	1,788.3
Total revenue	1,063.2	1,022.3	2,086.4	1,992.2
IAN | Total International
Total revenues:
Net Revenue	665.8	607.7	1,249.1	1,136.8
Total revenue	749.3	689.0	1,411.6	1,286.9
CMG
Total revenues:
Net Revenue	319.1	300.6	611.8	584.8
Total revenue	579.3	474.5	1,062.9	970.5
CMG | United States
Total revenues:
Net Revenue	208.2	201.3	402.5	396.4
Total revenue	382.5	328.5	710.0	673.8
CMG | Total International
Total revenues:
Net Revenue	110.9	99.3	209.3	188.4
Total revenue	$ 196.8	$ 146.0	$ 352.9	$ 296.7