Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
REVENUE	$ 3,578.1	$ 3,443.4	$ 10,631.3	$ 10,420.9
Media and production costs	761.5	678.4	761.5	678.4	$ 725.1
Contract assets and unbilled fees and costs	856.0	684.2	856.0	684.2	529.5
Contract assets:	1,617.5	1,362.6	1,617.5	1,362.6	1,254.6
Customer advances	1,192.9	1,358.8	1,192.9	1,358.8	$ 1,492.3
North America
Disaggregation of Revenue [Line Items]
REVENUE	1,983.2	1,969.0	5,888.8	5,777.5
UNITED STATES
Disaggregation of Revenue [Line Items]
REVENUE	1,868.8	1,847.8	5,531.6	5,415.2
Latin America
Disaggregation of Revenue [Line Items]
REVENUE	99.4	77.3	258.0	224.9
Europe
Disaggregation of Revenue [Line Items]
REVENUE	1,016.8	908.2	3,014.3	2,925.1
Middle East and Africa
Disaggregation of Revenue [Line Items]
REVENUE	51.6	62.0	199.1	208.8
Asia-Pacific
Disaggregation of Revenue [Line Items]
REVENUE	427.1	426.9	1,271.1	1,284.6
Advertising & Media
Disaggregation of Revenue [Line Items]
REVENUE	1,909.2	1,764.9	5,597.2	5,369.5
Precision Marketing
Disaggregation of Revenue [Line Items]
REVENUE	383.7	363.3	1,112.7	1,066.4
Commerce & Branding
Disaggregation of Revenue [Line Items]
REVENUE	211.6	211.9	631.7	630.3
Experiential
Disaggregation of Revenue [Line Items]
REVENUE	133.3	120.6	445.5	412.3
Execution & Support
Disaggregation of Revenue [Line Items]
REVENUE	206.1	262.0	673.2	811.2
Public Relations
Disaggregation of Revenue [Line Items]
REVENUE	392.4	393.0	1,161.5	1,149.4
Healthcare
Disaggregation of Revenue [Line Items]
REVENUE	$ 341.8	$ 327.7	$ 1,009.5	$ 981.8