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Acquisition (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2016
Dec. 31, 2015
Business Acquisition [Line Items]      
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low $ 100,000 $ 100,000 $ 100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High 15,700,000 15,700,000 $ 10,000,000
CRT business      
Business Acquisition [Line Items]      
Payment to acquire business   6,300,000  
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low 0 0  
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High 8,000,000 8,000,000  
Business Acquisition, Transaction Costs 400,000 400,000  
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual 3,500,000    
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual 400,000    
Cowen Prime and Cowen Prime Trading      
Business Acquisition [Line Items]      
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual 10,700,000 22,100,000  
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual $ 1,000,000 2,500,000  
Revenues   262,329,000  
Net income (loss) attributable to Cowen Group, Inc. common stockholders   $ 22,048,000  
Net income per common share - Basic (in dollars per share)   $ 0.20  
Net income per common share - Diluted (in dollars per share)   $ 0.19