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Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Salary and service costs                 $ 11,350.0omc_SalaryAndServiceCosts $ 10,724.4omc_SalaryAndServiceCosts $ 10,406.8omc_SalaryAndServiceCosts
Office and general expenses                 2,023.7omc_OfficeAndGeneralExpenses 2,034.8omc_OfficeAndGeneralExpenses 2,008.4omc_OfficeAndGeneralExpenses
Merger expenses     1.8us-gaap_BusinessCombinationAcquisitionRelatedCosts 7.0us-gaap_BusinessCombinationAcquisitionRelatedCosts 13.3us-gaap_BusinessCombinationAcquisitionRelatedCosts 28.1us-gaap_BusinessCombinationAcquisitionRelatedCosts     8.8us-gaap_BusinessCombinationAcquisitionRelatedCosts 41.4us-gaap_BusinessCombinationAcquisitionRelatedCosts  
Operating expenses 3,615.7us-gaap_OperatingExpenses 3,316.0us-gaap_OperatingExpenses 3,322.5us-gaap_OperatingExpenses 3,119.5us-gaap_OperatingExpenses 3,506.9us-gaap_OperatingExpenses 3,111.1us-gaap_OperatingExpenses 3,114.0us-gaap_OperatingExpenses 3,027.2us-gaap_OperatingExpenses 13,373.7us-gaap_OperatingExpenses 12,759.2us-gaap_OperatingExpenses 12,415.2us-gaap_OperatingExpenses
Available-for-sale securities 4.9us-gaap_AvailableForSaleSecurities       4.9us-gaap_AvailableForSaleSecurities       4.9us-gaap_AvailableForSaleSecurities 4.9us-gaap_AvailableForSaleSecurities  
Cost method investments 21.8us-gaap_CostMethodInvestments       22.2us-gaap_CostMethodInvestments       21.8us-gaap_CostMethodInvestments 22.2us-gaap_CostMethodInvestments  
Net foreign currency transaction gains (losses)                 $ 8.7us-gaap_ForeignCurrencyTransactionGainLossBeforeTax $ 2.7us-gaap_ForeignCurrencyTransactionGainLossBeforeTax $ (2.7)us-gaap_ForeignCurrencyTransactionGainLossBeforeTax
Percentage of revenue from largest client                 2.60%omc_PercentageOfRevenueFromLargestClient    
Percentage of revenue from any client other than largest, maximum                 2.50%omc_PercentageOfRevenueFromAnyClientOtherThanLargestMaximum    
Minimum [Member]                      
Identifiable intangible assets, useful life                 5 years    
Maximum [Member]                      
Identifiable intangible assets, useful life                 10 years    
Furniture [Member] | Minimum [Member]                      
Property and Equipment, useful life                 7 years    
Furniture [Member] | Maximum [Member]                      
Property and Equipment, useful life                 10 years    
Equipment [Member] | Minimum [Member]                      
Property and Equipment, useful life                 3 years    
Equipment [Member] | Maximum [Member]                      
Property and Equipment, useful life                 5 years