Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 4,015.6	$ 3,853.8	$ 7,706.0	$ 7,484.3
Media and production costs	889.6	843.9	889.6	843.9	$ 864.0
Unbilled fees and costs and contract assets	1,106.3	956.1	1,106.3	956.1	758.2
Work in process	1,995.9	1,800.0	1,995.9	1,800.0	1,622.2
Customer advances	1,348.2	1,262.1	1,348.2	1,262.1	$ 1,336.1
North America
Disaggregation of Revenue [Line Items]
Revenue	2,209.7	2,148.4	4,321.2	4,189.3
United States
Disaggregation of Revenue [Line Items]
Revenue	2,093.5	2,033.4	4,100.5	3,959.3
Latin America
Disaggregation of Revenue [Line Items]
Revenue	114.6	106.4	211.0	202.9
Europe
Disaggregation of Revenue [Line Items]
Revenue	1,166.4	1,101.9	2,161.4	2,107.7
Middle East and Africa
Disaggregation of Revenue [Line Items]
Revenue	66.1	65.6	136.9	145.2
Asia-Pacific
Disaggregation of Revenue [Line Items]
Revenue	458.8	431.5	875.5	839.2
Media & Advertising
Disaggregation of Revenue [Line Items]
Revenue	2,291.3	2,101.0	4,339.5	4,055.2
Precision Marketing
Disaggregation of Revenue [Line Items]
Revenue	457.1	427.1	907.4	852.5
Public Relations
Disaggregation of Revenue [Line Items]
Revenue	372.9	407.9	735.6	791.4
Healthcare
Disaggregation of Revenue [Line Items]
Revenue	332.6	345.1	638.3	662.1
Branding & Retail Commerce
Disaggregation of Revenue [Line Items]
Revenue	148.6	182.1	308.1	367.6
Experiential
Disaggregation of Revenue [Line Items]
Revenue	196.8	182.4	351.0	338.1
Execution & Support
Disaggregation of Revenue [Line Items]
Revenue	$ 216.3	$ 208.2	$ 426.1	$ 417.4