Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
REVENUE	$ 3,443.3	$ 3,410.3
Media and production costs	710.9	654.3	$ 725.1
Contract assets and unbilled fees and costs	798.0	662.5	529.5
Contract assets:	1,508.9	1,316.8	1,254.6
Customer advances	1,279.6	1,463.8	$ 1,492.3
North America
Disaggregation of Revenue [Line Items]
REVENUE	1,926.8	1,839.0
UNITED STATES
Disaggregation of Revenue [Line Items]
REVENUE	1,812.2	1,724.6
Latin America
Disaggregation of Revenue [Line Items]
REVENUE	74.0	67.7
Europe
Disaggregation of Revenue [Line Items]
REVENUE	951.9	992.0
Middle East and Africa
Disaggregation of Revenue [Line Items]
REVENUE	84.9	81.9
Asia-Pacific
Disaggregation of Revenue [Line Items]
REVENUE	405.7	429.7
Advertising & Media
Disaggregation of Revenue [Line Items]
REVENUE	1,776.5	1,770.2
Precision Marketing
Disaggregation of Revenue [Line Items]
REVENUE	360.0	339.2
Commerce & Brand Consulting
Disaggregation of Revenue [Line Items]
REVENUE	209.6	211.4
Experiential
Disaggregation of Revenue [Line Items]
REVENUE	147.8	139.7
Execution & Support
Disaggregation of Revenue [Line Items]
REVENUE	255.5	277.0
Public Relations
Disaggregation of Revenue [Line Items]
REVENUE	375.5	362.4
Healthcare
Disaggregation of Revenue [Line Items]
REVENUE	$ 318.4	$ 310.4