Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 4,037.1	$ 3,882.6	$ 11,743.1	$ 11,366.9
Media and production costs	908.3	881.1	908.3	881.1	$ 864.0
Unbilled fees and costs and contract assets	1,115.2	1,017.3	1,115.2	1,017.3	758.2
Work in process	2,023.5	1,898.4	2,023.5	1,898.4	1,622.2
Customer advances	1,273.0	1,303.2	1,273.0	1,303.2	$ 1,336.1
North America
Disaggregation of Revenue [Line Items]
Revenue	2,245.3	2,154.0	6,566.5	6,343.3
United States
Disaggregation of Revenue [Line Items]
Revenue	2,132.9	2,039.7	6,233.4	5,999.0
Latin America
Disaggregation of Revenue [Line Items]
Revenue	126.4	99.7	337.4	302.6
Europe
Disaggregation of Revenue [Line Items]
Revenue	1,135.4	1,080.8	3,296.8	3,188.5
Middle East and Africa
Disaggregation of Revenue [Line Items]
Revenue	67.4	63.3	204.3	208.5
Asia-Pacific
Disaggregation of Revenue [Line Items]
Revenue	462.6	484.8	1,338.1	1,324.0
Media & Advertising
Disaggregation of Revenue [Line Items]
Revenue	2,353.8	2,129.0	6,693.3	6,184.2
Precision Marketing
Disaggregation of Revenue [Line Items]
Revenue	462.5	450.6	1,369.9	1,303.1
Public Relations
Disaggregation of Revenue [Line Items]
Revenue	377.2	404.0	1,112.8	1,195.3
Healthcare
Disaggregation of Revenue [Line Items]
Revenue	331.2	337.1	969.5	999.1
Branding & Retail Commerce
Disaggregation of Revenue [Line Items]
Revenue	144.8	181.7	452.9	549.3
Experiential
Disaggregation of Revenue [Line Items]
Revenue	152.2	174.3	503.2	512.5
Execution & Support
Disaggregation of Revenue [Line Items]
Revenue	$ 215.4	$ 205.9	$ 641.5	$ 623.4