Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total revenue	$ 2,856.0	$ 2,297.5	$ 2,391.8	$ 2,169.1	$ 2,589.8	$ 2,208.2	$ 2,185.8	$ 2,063.8	$ 9,714.4	$ 9,047.6	$ 9,056.2
Net revenue	$ 2,413.7	$ 1,895.7	$ 1,948.2	$ 1,774.0	$ 2,131.1	$ 1,832.5	$ 1,834.6	$ 1,675.3	8,031.6	7,473.5	7,452.3
United States
Total revenue									5,851.0	5,417.3	5,452.6
Net revenue									4,825.0	4,458.8	4,443.2
Total International
Total revenue									3,863.4	3,630.3	3,603.6
Net revenue									3,206.6	3,014.7	3,009.1
United Kingdom
Total revenue									881.4	775.7	788.8
Net revenue									711.7	613.1	604.3
Continental Europe
Total revenue									840.2	780.6	764.1
Net revenue									737.5	687.8	682.0
Asia Pacific
Total revenue									1,170.8	1,106.4	1,076.0
Net revenue									896.8	866.9	887.7
Latin America
Total revenue									389.0	386.6	417.2
Net revenue									350.1	350.8	367.8
Other
Total revenue									582.0	581.0	557.5
Net revenue									510.5	496.1	467.3
CMG
Total revenue									2,158.3	2,038.0	2,063.4
Net revenue									1,264.1	1,206.8	1,250.9
CMG | United States
Total revenue									1,404.0	1,355.0	1,376.7
Net revenue									824.6	798.2	842.9
CMG | Total International
Total revenue									754.3	683.0	686.7
Net revenue									439.5	408.6	408.0
IAN
Total revenue									7,374.4	7,009.6	6,992.8
Net revenue									6,585.8	6,266.7	6,201.4
IAN | United States
Total revenue									4,279.4	4,062.3	4,075.9
Net revenue									3,832.7	3,660.6	3,600.3
IAN | Total International
Total revenue									3,095.0	2,947.3	2,916.9
Net revenue									$ 2,753.1	$ 2,606.1	$ 2,601.1