Document and Entity Information	9 Months Ended
	Sep. 30, 2012	Nov. 07, 2012
Document and Entity Information [Abstract]
Entity Registrant Name	Cowen Group, Inc.
Entity Central Index Key	0001466538
Current Fiscal Year End Date	--12-31
Document Period End Date	Sep. 30, 2012
Entity Filer Category	Accelerated Filer
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q3
Document Type	10-Q
Amendment Flag	false
Entity Common Stock, Shares Outstanding		113,508,235