Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 11, 2021	Jun. 30, 2020
Entity Listings [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Entity File Number	1-10551
Entity Registrant Name	OMNICOM GROUP INC.
Entity Incorporation, State or Country Code	NY
Entity Tax Identification Number	13-1514814
Entity Address, Address Line One	280 Park Avenue
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10017
City Area Code	212
Local Phone Number	415-3600
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 11,583,494,559
Entity Common Stock, Shares Outstanding		215,014,390
Entity Central Index Key	0000029989
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Document Transition Report	false
Amendment Flag	false
Common Stock, $0.15 Par Value
Entity Listings [Line Items]
Title of 12(b) Security	Common Stock, $0.15 Par Value
Trading Symbol	OMC
Security Exchange Name	NYSE
0.800% Senior Notes due 2027
Entity Listings [Line Items]
Title of 12(b) Security	0.800% Senior Notes due 2027
Trading Symbol	OMC/27
Security Exchange Name	NYSE
1.400% Senior Notes due 2031
Entity Listings [Line Items]
Title of 12(b) Security	1.400% Senior Notes due 2031
Trading Symbol	OMC/31
Security Exchange Name	NYSE