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Goodwill and Intangible Assets Goodwill Details (Details) (USD $)
12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 31, 2013
Goodwill [Line Items]        
Goodwill impairment $ 2,334,000us-gaap_GoodwillImpairmentLoss $ 0us-gaap_GoodwillImpairmentLoss $ 0us-gaap_GoodwillImpairmentLoss  
Dahlman Rose & Company, LLC        
Goodwill [Line Items]        
Goodwill, Other Changes       8,700,000us-gaap_GoodwillOtherChanges
/ us-gaap_BusinessAcquisitionAxis
= cown_DahlmanRoseCompanyLlcMember
Broker-Dealer        
Goodwill [Line Items]        
Goodwill impairment $ 2,334,000us-gaap_GoodwillImpairmentLoss
/ us-gaap_StatementBusinessSegmentsAxis
= cown_BrokerDealerMember
$ 0us-gaap_GoodwillImpairmentLoss
/ us-gaap_StatementBusinessSegmentsAxis
= cown_BrokerDealerMember