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Share-Based Compensation and Employee Ownership Plans RCG Grants (Details) (RCG Grants, USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
RCG Grants
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Allocated Share-based Compensation Expense $ 4.9us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= cown_RCGGrantsMember
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense $ 1.9us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_PlanNameAxis
= cown_RCGGrantsMember