Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
REVENUE	$ 3,609.9	$ 3,567.2	$ 7,053.2	$ 6,977.5
Media and production costs	797.1	588.5	797.1	588.5	$ 725.1
Contract assets and unbilled fees and costs	840.9	647.4	840.9	647.4	529.5
Contract assets:	1,638.0	1,235.9	1,638.0	1,235.9	1,254.6
Customer advances	1,159.7	1,372.8	1,159.7	1,372.8	$ 1,492.3
North America
Disaggregation of Revenue [Line Items]
REVENUE	1,978.8	1,969.5	3,905.6	3,808.5
UNITED STATES
Disaggregation of Revenue [Line Items]
REVENUE	1,850.6	1,842.8	3,662.8	3,567.4
Latin America
Disaggregation of Revenue [Line Items]
REVENUE	84.6	79.9	158.6	147.6
Europe
Disaggregation of Revenue [Line Items]
REVENUE	1,045.6	1,024.9	1,997.5	2,016.9
Middle East and Africa
Disaggregation of Revenue [Line Items]
REVENUE	62.6	64.9	147.5	146.8
Asia-Pacific
Disaggregation of Revenue [Line Items]
REVENUE	438.3	428.0	844.0	857.7
Advertising & Media
Disaggregation of Revenue [Line Items]
REVENUE	1,911.5	1,834.4	3,688.0	3,604.6
Precision Marketing
Disaggregation of Revenue [Line Items]
REVENUE	369.0	363.9	729.0	703.1
Commerce & Brand Consulting
Disaggregation of Revenue [Line Items]
REVENUE	210.5	207.0	420.1	418.4
Experiential
Disaggregation of Revenue [Line Items]
REVENUE	164.4	152.0	312.2	291.7
Execution & Support
Disaggregation of Revenue [Line Items]
REVENUE	211.6	272.2	467.1	549.2
Public Relations
Disaggregation of Revenue [Line Items]
REVENUE	393.6	394.0	769.1	756.4
Healthcare
Disaggregation of Revenue [Line Items]
REVENUE	$ 349.3	$ 343.7	$ 667.7	$ 654.1