Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Total revenues:
Net revenue	$ 1,895.7	$ 1,832.5	$ 5,617.9	$ 5,342.4
Total revenue	2,297.5	2,208.2	6,858.4	6,457.8
United States
Total revenues:
Net revenue	1,160.9	1,109.6	3,424.7	3,294.3
Total revenue	1,407.1	1,340.8	4,203.5	4,006.8
Total International
Total revenues:
Net revenue	734.8	722.9	2,193.2	2,048.1
Total revenue	890.4	867.4	2,654.9	2,451.0
United Kingdom
Total revenues:
Net revenue	174.0	160.0	513.2	436.9
Total revenue	220.9	201.7	626.9	542.7
Continental Europe
Total revenues:
Net revenue	152.0	150.7	489.4	446.2
Total revenue	177.5	171.6	564.3	506.1
Asia Pacific
Total revenues:
Net revenue	210.6	203.7	603.6	582.7
Total revenue	267.3	259.9	803.0	740.7
Latin America
Total revenues:
Net revenue	84.0	86.9	239.9	241.6
Total revenue	95.3	94.9	267.8	265.2
Other
Total revenues:
Net revenue	114.2	121.6	347.1	340.7
Total revenue	129.4	139.3	392.9	396.3
IAN
Total revenues:
Net revenue	1,580.5	1,531.4	4,690.9	4,456.5
Total revenue	1,774.7	1,702.8	5,272.7	4,981.9
IAN | United States
Total revenues:
Net revenue	952.9	913.1	2,814.2	2,701.4
Total revenue	1,064.2	1,006.2	3,150.6	2,998.4
IAN | Total International
Total revenues:
Net revenue	627.6	618.3	1,876.7	1,755.1
Total revenue	710.5	696.6	2,122.1	1,983.5
CMG
Total revenues:
Net revenue	315.2	301.1	927.0	885.9
Total revenue	522.8	505.4	1,585.7	1,475.9
CMG | United States
Total revenues:
Net revenue	208.0	196.5	610.5	592.9
Total revenue	342.9	334.6	1,052.9	1,008.4
CMG | Total International
Total revenues:
Net revenue	107.2	104.6	316.5	293.0
Total revenue	$ 179.9	$ 170.8	$ 532.8	$ 467.5