Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Net Income Available for Common Shares:
Net income - Omnicom Group Inc.	$ 209.1us-gaap_NetIncomeLoss	$ 205.5us-gaap_NetIncomeLoss
Net income allocated to participating securities	2.8	4.1
Net income available for common shares	$ 206.3	$ 201.4
Weighted Average Shares:
Basic	246.4	259.1
Dilutive stock options and restricted shares	1.0	2.3
Diluted	247.4	261.4
Anti-dilutive stock options and restricted shares	0.1	0.1
Net Income per Common Share - Omnicom Group Inc.:
Basic	$ 0.84us-gaap_EarningsPerShareBasic	$ 0.78us-gaap_EarningsPerShareBasic
Diluted	$ 0.83us-gaap_EarningsPerShareDiluted	$ 0.77us-gaap_EarningsPerShareDiluted

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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