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Acquisition (Details) - Cowen Prime and Cowen Prime Trading
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2016
USD ($)
$ / shares
Business Acquisition [Line Items]  
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual $ 11,400
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual 1,800
Revenues 132,363
Net income (loss) attributable to Cowen Group, Inc. common stockholders $ 16,547
Net income per common share - Basic (in dollars per share) | $ / shares $ 0.15
Net income per common share - Diluted (in dollars per share) | $ / shares $ 0.14