Document and Entity Information
9 Months Ended
Sep. 30, 2014
Nov. 05, 2014
Document and Entity Information [Abstract]    
Entity Registrant Name Cowen Group, Inc.  
Entity Central Index Key 0001466538  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Document Type 10-Q  
Document Period End Date Sep. 30, 2014  
Document Fiscal Year Focus 2014  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   113,635,176