| Cover | Feb. 18, 2021 |
| Schedule of Capitalization, Equity [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 18, 2021 |
| Entity File Number | 1-10551 |
| Entity Registrant Name | OMNICOM GROUP INC. |
| Entity Central Index Key | 0000029989 |
| Entity Tax Identification Number | 13-1514814 |
| Entity Incorporation, State or Country Code | NY |
| Entity Address, Address Line One | 280 Park Avenue |
| Entity Address, City or Town | NY |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10017 |
| City Area Code | 212 |
| Local Phone Number | 415-3600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Information, Former Legal or Registered Name | Not Applicable |
| Common Stock, par value $0.15 per share | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | Common Stock, par value $0.15 per share |
| Trading Symbol | OMC |
| Security Exchange Name | NYSE |
| 0.800% Senior Notes due 2027 | |
| Schedule of Capitalization, Equity [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 | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | 1.400% Senior Notes due 2031 |
| Trading Symbol | OMC/31 |
| Security Exchange Name | NYSE |
