| Statement of Financial Position, Classified (USD $) | ||
| In Millions, except Per Share data | Dec. 31, 2008
| Dec. 31, 2007
|
| Cash and Cash Equivalents, at Carrying Value [Abstract] | ||
| Cash and Cash Equivalents, at Carrying Value, Total | 1097.3 | 1793.2 |
| Short-term Investments [Abstract] | ||
| Short-term Investments, Total | 15.1 | 47.8 |
| Accounts Receivable, Net, Current [Abstract] | ||
| Accounts Receivable, Gross, Current | 5835.4 | 6885.1 |
| Allowance for Doubtful Accounts Receivable, Current | 59.9 | 54.7 |
| Accounts Receivable, Net, Current, Total | 5775.5 | 6830.4 |
| Inventory, Net [Abstract] | ||
| Inventory, Work in Process | 672 | 801 |
| Other Assets, Current | 1,005 | 1031.8 |
| Assets, Current, Total | 8564.9 | 10504.2 |
| Property, Plant and Equipment, Gross [Abstract] | ||
| Property, Plant and Equipment, Gross, Total | 1750.7 | 1766.5 |
| Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment | 1031.1 | 1059.8 |
| Property, Plant and Equipment, Net, Total | 719.6 | 706.7 |
| Investments in Affiliates, Subsidiaries, Associates, and Joint Ventures [Abstract] | ||
| Investments in Affiliates, Subsidiaries, Associates, and Joint Ventures, Total | 297.3 | 247.1 |
| Goodwill | 7220.2 | 7318.5 |
| Finite-Lived Intangible Assets, Gross [Abstract] | ||
| Finite-Lived Intangible Assets, Gross, Total | 499.4 | 447.3 |
| Finite-Lived Intangible Assets, Accumulated Amortization | 278.4 | 251.6 |
| Intangible Assets, Net (Excluding Goodwill), Total | 221 | 195.7 |
| Deferred Tax Assets, Net, Noncurrent | 45.2 | 40.5 |
| Other Assets, Noncurrent | 250.2 | 259 |
| Assets, Total | 17318.4 | 19271.7 |
| Accounts Payable and Accrued Liabilities [Abstract] | ||
| Accounts Payable | 6881.2 | 8080.5 |
| Customer Advances and Deposits, Current [Abstract] | ||
| Customer Advances, Current | 1005.5 | 1122.8 |
| Long-term Debt, Current [Abstract] | ||
| Long-term Debt, Current, Total | 2.7 | 2.6 |
| Short-term Borrowings [Abstract] | ||
| Short-term Borrowings, Total | 16.2 | 12 |
| Taxes Payable [Abstract] | ||
| Taxes Payable, Total | 201.1 | 250.7 |
| Other Liabilities, Current | 1647.5 | 1758.6 |
| Liabilities, Current, Total | 9754.2 | 11227.2 |
| Long-term Debt, Noncurrent [Abstract] | ||
| Long-term Debt, Noncurrent, Total | 1012.8 | 1013.2 |
| Convertible Debt, Noncurrent | 2041.5 | 2041.5 |
| Other Liabilities, Noncurrent | 444.4 | 481.2 |
| Deferred Tax Liabilities, Noncurrent | 312.1 | 174.8 |
| Minority Interest [Abstract] | ||
| Minority Interest, Total | 230.6 | 242.1 |
| Stockholders' Equity [Abstract] | ||
| Preferred Stock, Value | 0 | 0 |
| Preferred Stock, Par or Stated Value Per Share | $1 | $1 |
| Preferred Stock, Shares Authorized | 7.5 | 7.5 |
| Preferred Stock, Shares Issued | 0 | 0 |
| Common Stock, Value | 59.6 | 59.6 |
| Common Stock, Par or Stated Value Per Share | 0.15 | 0.15 |
| Common Stock, Shares Authorized | 1,000 | 1,000 |
| Common Stock, Shares, Issued | 397.2 | 397.2 |
| Common Stock, Shares, Outstanding | 307.3 | 323 |
| Additional Paid in Capital [Abstract] | ||
| Additional Paid in Capital, Common Stock | 1600.5 | 1619.5 |
| Retained Earnings (Accumulated Deficit) [Abstract] | ||
| Retained Earnings (Accumulated Deficit), Total | 5888.1 | 5077.5 |
| Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] | ||
| Accumulated Other Comprehensive Income (Loss), Net of Tax, Total | -247.3 | 430.7 |
| Treasury Stock, Value | -3778.1 | -3095.6 |
| Treasury Stock, Shares | 89.9 | 74.2 |
| Stockholders' Equity, Total | 3522.8 | 4091.7 |
| Liabilities and Stockholders' Equity, Total | 17318.4 | 19271.7 |
| Statement of Income (Excluding Gross Margin Alternative) (USD $) | |||
| In Millions, except Per Share data | 12 Months Ended
Dec. 31, 2008 | 12 Months Ended
Dec. 31, 2007 | 12 Months Ended
Dec. 31, 2006 |
| Revenues [Abstract] | |||
| Revenues, Total | 13359.9 | $12,694 | 11376.9 |
| Operating Expenses [Abstract] | |||
| Operating Expenses, Total | 11670.5 | 11034.9 | 9893.4 |
| Operating Income (Loss), Total | 1689.4 | 1659.1 | 1483.5 |
| Interest Expense [Abstract] | |||
| Interest Expense, Total | 124.6 | 106.9 | 124.9 |
| Investment Income, Interest and Dividend [Abstract] | |||
| Investment Income, Interest | -50.3 | -32.9 | -33.3 |
| Income (Loss) from Continuing Operations before Income Taxes, Minority Interest, and Income (Loss) from Equity Method Investments, Total | 1615.1 | 1585.1 | 1391.9 |
| Income Tax Expense (Benefit) [Abstract] | |||
| Income Tax Expense (Benefit), Total | 542.7 | 536.9 | 466.9 |
| Income (Loss) from Equity Method Investments | 42 | 38.4 | 29.6 |
| Minority Interest in Net Income (Loss) of Consolidated Entities [Abstract] | |||
| Minority Interest in Net Income (Loss) of Consolidated Entities, Total | -114.1 | -110.9 | -90.6 |
| Net Income (Loss), Total | 1000.3 | 975.7 | $864 |
| Earnings Per Share, Basic [Abstract] | |||
| Earnings Per Share, Basic, Total | 3.2 | 2.99 | 2.52 |
| Earnings Per Share, Diluted [Abstract] | |||
| Earnings Per Share, Diluted, Total | 3.17 | 2.95 | 2.5 |
| Statement of Shareholders' Equity and Other Comprehensive Income (USD $) | |||||||
| In Millions, except Share data | Common Stock [Member]
| Common Stock at Par Value [Member]
| Additional Paid-in Capital [Member]
| Treasury Stock [Member]
| Retained Earnings [Member]
| Accumulated Other Comprehensive Income [Member]
| Total
|
| Stockholders' Equity, Beginning Balance at Dec. 31, 2005 | 29.8 | 1675.1 | -1415.7 | $3,599 | 59.8 | $3,948 | |
| Shares, Issued, Beginning Balance at Dec. 31, 2005 | 198,629,712 | ||||||
| Net Income (Loss) [Abstract] | |||||||
| Net Income (Loss), Total | 864 | 864 | |||||
| Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Net of Tax, Period Increase (Decrease) [Abstract] | |||||||
| Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Net of Tax, Period Increase (Decrease), Total | 239.1 | 239.1 | |||||
| Cumulative Effect of Initial Adoption of New Accounting Principle [Abstract] | |||||||
| Cumulative Effect of Initial Adoption of SFAS 158 | (31) | (31) | |||||
| Dividends, Common Stock | -173.2 | -173.2 | |||||
| Share-based Compensation [Abstract] | |||||||
| Share-based Compensation, Total | 71.1 | 71.1 | |||||
| Stock Issued During Period, Value, Share-based Compensation | -83.9 | 381.8 | 297.9 | ||||
| Treasury Stock, Value, Acquired, Cost Method | -1344.6 | -1344.6 | |||||
| Cancellation of Common Stock, Value | -0.2 | -0.2 | 0 | ||||
| Cancellation of Common Stock, Shares | (2,485) | ||||||
| Stockholders' Equity, Ending Balance at Dec. 31, 2006 | 29.8 | 1662.1 | -2378.3 | 4289.8 | 267.9 | 3871.3 | |
| Shares, Issued, Ending Balance at Dec. 31, 2006 | 198,627,227 | ||||||
| Net Income (Loss) [Abstract] | |||||||
| Net Income (Loss), Total | 975.7 | 975.7 | |||||
| Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Net of Tax, Period Increase (Decrease) [Abstract] | |||||||
| Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Net of Tax, Period Increase (Decrease), Total | 163.5 | 163.5 | |||||
| Other Comprehensive Income, Defined Benefit Plans Adjustment, Net of Tax, Period Increase (Decrease) [Abstract] | |||||||
| Other Comprehensive Income, Defined Benefit Plans Adjustment, Net of Tax, Period Increase (Decrease), Total | -0.7 | -0.7 | |||||
| Cumulative Effect of Initial Adoption of New Accounting Principle [Abstract] | |||||||
| Cumulative Effect of Initial Adoption of FIN 48 | 1.3 | 1.3 | |||||
| Stock Issued During Period, Value, Stock Dividend | 29.8 | -29.8 | 0 | ||||
| Stock Issued During Period, Shares, Stock Dividend | 198,627,227 | ||||||
| Dividends, Common Stock | -189.3 | -189.3 | |||||
| Share-based Compensation [Abstract] | |||||||
| Share-based Compensation, Total | 68.7 | 68.7 | |||||
| Stock Issued During Period, Value, Share-based Compensation | -80.2 | 181.1 | 100.9 | ||||
| Treasury Stock, Value, Acquired, Cost Method | -899.7 | -899.7 | |||||
| Cancellation of Common Stock, Value | -1.3 | 1.3 | 0 | ||||
| Cancellation of Common Stock, Shares | (29,372) | ||||||
| Stockholders' Equity, Ending Balance at Dec. 31, 2007 | 59.6 | 1619.5 | -3095.6 | 5077.5 | 430.7 | 4091.7 | |
| Shares, Issued, Ending Balance at Dec. 31, 2007 | 397,225,082 | ||||||
| Net Income (Loss) [Abstract] | |||||||
| Net Income (Loss), Total | 1000.3 | 1000.3 | |||||
| Other Comprehensive Income, Available-for-sale Securities Adjustment, Net of Tax, Period Increase (Decrease) [Abstract] | |||||||
| Other Comprehensive Income, Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax | (12) | (12) | |||||
| Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Net of Tax, Period Increase (Decrease) [Abstract] | |||||||
| Other Comprehensive Income, Foreign Currency Transaction and Translation Adjustment, Net of Tax, Period Increase (Decrease), Total | -654.9 | -654.9 | |||||
| Other Comprehensive Income, Defined Benefit Plans Adjustment, Net of Tax, Period Increase (Decrease) [Abstract] | |||||||
| Other Comprehensive Income, Defined Benefit Plans Adjustment, Net of Tax, Period Increase (Decrease), Total | -11.1 | -11.1 | |||||
| Cumulative Effect of Initial Adoption of New Accounting Principle [Abstract] | |||||||
| Dividends, Common Stock | -189.7 | -189.7 | |||||
| Share-based Compensation [Abstract] | |||||||
| Share-based Compensation, Total | 59.3 | 59.3 | |||||
| Stock Issued During Period, Value, Share-based Compensation | -78.2 | 164.2 | 86 | ||||
| Treasury Stock, Value, Acquired, Cost Method | -846.8 | -846.8 | |||||
| Cancellation of Common Stock, Value | -0.1 | 0.1 | 0 | ||||
| Cancellation of Common Stock, Shares | (2,004) | ||||||
| Stockholders' Equity, Ending Balance at Dec. 31, 2008 | 59.6 | 1600.5 | -3778.1 | 5888.1 | -247.3 | 3522.8 | |
| Shares, Issued, Ending Balance at Dec. 31, 2008 | 397,223,078 |
| Statement of Cash Flows (USD $) | |||
| In Millions | 12 Months Ended
Dec. 31, 2008 | 12 Months Ended
Dec. 31, 2007 | 12 Months Ended
Dec. 31, 2006 |
| Income (Loss) from Continuing Operations [Abstract] | |||
| Net Income (Loss) | 1000.3 | 975.7 | $864 |
| Depreciation [Abstract] | |||
| Depreciation, Total | 182.8 | 164.2 | 150.3 |
| Adjustment for Amortization [Abstract] | |||
| Amortization of Intangible Assets | 53.1 | 44.4 | 39.7 |
| Minority Interest in Net Income (Loss) of Consolidated Entities | 114.1 | 110.9 | 90.6 |
| Income (Loss) from Equity Method Investments, Net of Dividends or Distributions [Abstract] | |||
| Income (Loss) from Equity Method Investments, Net of Dividends or Distributions, Total | -14.7 | (10) | -15.3 |
| Gain (Loss) on Investments [Abstract] | |||
| Gain (Loss) on Investments, Total | -2.3 | -2.4 | -7.8 |
| Excess Tax Benefit from Share-based Compensation, Operating Activities | -12.9 | -17.2 | -26.6 |
| Provision for Doubtful Accounts | 26.5 | 21.2 | 10.7 |
| Share-based Compensation [Abstract] | |||
| Share-based Compensation, Total | 59.3 | 68.7 | 71.1 |
| Increase (Decrease) in Operating Capital [Abstract] | |||
| Increase (Decrease) in Operating Capital, Total | (12) | 243.8 | 564.5 |
| Net Cash Provided by (Used in) Operating Activities, Total | 1394.2 | 1599.3 | 1741.2 |
| Payments to Acquire Property, Plant, and Equipment [Abstract] | |||
| Payments to Acquire Property, Plant, and Equipment, Total | -212.2 | (223) | -177.6 |
| Payments to Acquire Businesses and Interest in Affiliates [Abstract] | |||
| Payments to Acquire Businesses and Interest in Affiliates, Total | -441.4 | -358.8 | -236.3 |
| Proceeds from Sale of Short-term Investments | 37.5 | 183.3 | 530.4 |
| Payments to Acquire Investments [Abstract] | |||
| Payments to Acquire Short-term Investments | -13.1 | (42) | -350.2 |
| Proceeds from Sale and Collection of Notes Receivables [Abstract] | |||
| Proceeds from Collection of Notes Receivable | 0 | 0 | 13.5 |
| Proceeds from Divestiture of Businesses and Interests in Affiliates [Abstract] | |||
| Proceeds from Divestiture of Businesses and Interests in Affiliates, Total | 0 | 0 | 31.4 |
| Payments for (Proceeds from) Other Investing Activities | -50.8 | 0 | 0 |
| Net Cash Provided by (Used in) Investing Activities, Total | (680) | -440.5 | -188.8 |
| Proceeds from (Repayments of) Short-term Debt [Abstract] | |||
| Proceeds from (Repayments of) Short-term Debt, Total | 5.1 | -0.9 | -3.5 |
| Proceeds from Issuance of Long-term Debt [Abstract] | |||
| Proceeds from Issuance of Long-term Debt, Total | 2.4 | 3.4 | 996.6 |
| Repayments of Long-term Debt [Abstract] | |||
| Repayments of Long-term Debt, Total | (2) | (2) | -300.4 |
| Excess Tax Benefit from Share-based Compensation, Financing Activities | 12.9 | 17.2 | 26.6 |
| Payments of Dividends [Abstract] | |||
| Payments of Dividends, Total | (192) | -182.8 | -175.8 |
| Payments for Repurchase of Equity [Abstract] | |||
| Payments for Repurchase of Common Stock | -846.8 | -899.7 | -1368.2 |
| Proceeds from Issuance or Sale of Equity [Abstract] | |||
| Proceeds from Stock Plans | 86 | 100.9 | 321.5 |
| Proceeds from (Payments for) Other Financing Activities | -119.4 | -76.8 | -80.3 |
| Net Cash Provided by (Used in) Financing Activities, Total | -1053.8 | -1040.7 | -583.5 |
| Effect of Exchange Rate on Cash and Cash Equivalents [Abstract] | |||
| Effect of Exchange Rate on Cash and Cash Equivalents, Total | -356.3 | -64.4 | -65.2 |
| Cash and Cash Equivalents, Period Increase (Decrease), Total | -695.9 | 53.7 | 903.7 |
| Cash and Cash Equivalents, at Carrying Value, Beginning Balance | 1793.2 | 1739.5 | 835.8 |
| Cash and Cash Equivalents, at Carrying Value, Ending Balance | 1097.3 | 1793.2 | 1739.5 |
| Document Information | |
| 12 Months Ended
Dec. 31, 2008 USD / shares | |
| Document Information [Line Items] | |
| Document Type | 10-K |
| Amendment Flag | false |
| Document Period End Date | 2008-12-31 |
| Entity Information (USD $) | |
| In Millions, except Share data | 12 Months Ended
Dec. 31, 2008 |
| Entity Information [Line Items] | |
| Entity Registrant Name | OMNICOM GROUP INC. |
| Entity Central Index Key | 0000029989 |
| Current Fiscal Year End Date | --12-31 |
| Entity Well-known Seasoned Issuer | Yes |
| Entity Voluntary Filers | No |
| Entity Current Reporting Status | Yes |
| Entity Filer Category | Large Accelerated Filer |
| Entity Public Float | $14,325 |
| Entity Common Stock, Shares Outstanding | 310,932,000 |
