Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 3,690.4	$ 3,630.5
Media and production costs	899.2	788.8	$ 864.0
Unbilled fees and costs and contract assets	1,031.0	922.8	758.2
Work in process	1,930.2	1,711.6	1,622.2
Customer advances	1,283.5	1,238.2	$ 1,336.1
North America
Disaggregation of Revenue [Line Items]
Revenue	2,111.5	2,040.9
United States
Disaggregation of Revenue [Line Items]
Revenue	2,007.0	1,925.9
Latin America
Disaggregation of Revenue [Line Items]
Revenue	96.4	96.5
Europe
Disaggregation of Revenue [Line Items]
Revenue	995.0	1,005.8
Middle East and Africa
Disaggregation of Revenue [Line Items]
Revenue	70.8	79.6
Asia-Pacific
Disaggregation of Revenue [Line Items]
Revenue	416.7	407.7
Media & Advertising
Disaggregation of Revenue [Line Items]
Revenue	2,048.2	1,954.2
Precision Marketing
Disaggregation of Revenue [Line Items]
Revenue	450.3	425.4
Public Relations
Disaggregation of Revenue [Line Items]
Revenue	362.7	383.5
Healthcare
Disaggregation of Revenue [Line Items]
Revenue	305.7	317.0
Branding & Retail Commerce
Disaggregation of Revenue [Line Items]
Revenue	159.5	185.5
Experiential
Disaggregation of Revenue [Line Items]
Revenue	154.2	155.7
Execution & Support
Disaggregation of Revenue [Line Items]
Revenue	$ 209.8	$ 209.2