Revenue Major Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total revenue	$ 2,901.8	$ 2,438.1	$ 2,520.2	$ 2,361.2	$ 2,856.0	$ 2,297.5	$ 2,391.8	$ 2,169.1	$ 10,221.3	$ 9,714.4	$ 9,047.6
Net revenue	$ 2,433.0	$ 2,061.4	$ 2,125.9	$ 2,004.8	$ 2,413.7	$ 1,895.7	$ 1,948.2	$ 1,774.0	8,625.1	8,031.6	7,473.5
United States
Total revenue									6,368.7	5,851.0	5,417.3
Net revenue									5,386.1	4,825.0	4,458.8
Total International
Total revenue									3,852.6	3,863.4	3,630.3
Net revenue									3,239.0	3,206.6	3,014.7
United Kingdom
Total revenue									871.8	881.4	775.7
Net revenue									727.0	711.7	613.1
Continental Europe
Total revenue									842.9	840.2	780.6
Net revenue									742.4	737.5	687.8
Asia Pacific
Total revenue									1,102.3	1,170.8	1,106.4
Net revenue									858.3	896.8	866.9
Latin America
Total revenue									431.6	389.0	386.6
Net revenue									389.9	350.1	350.8
Other
Total revenue									604.0	582.0	581.0
Net revenue									521.4	510.5	496.1
CMG
Total revenue									2,194.9	2,158.3	2,038.0
Net revenue									1,276.9	1,264.1	1,206.8
CMG | United States
Total revenue									1,456.2	1,404.0	1,355.0
Net revenue									828.6	824.6	798.2
CMG | Total International
Total revenue									738.7	754.3	683.0
Net revenue									448.3	439.5	408.6
IAN
Total revenue									8,026.4	7,556.1	7,009.6
Net revenue									7,348.2	6,767.5	6,266.7
IAN | United States
Total revenue									4,912.5	4,447.0	4,062.3
Net revenue									4,557.5	4,000.4	3,660.6
IAN | Total International
Total revenue									3,113.9	3,109.1	2,947.3
Net revenue									$ 2,790.7	$ 2,767.1	$ 2,606.1