Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2016	Jul. 29, 2016
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	Jun. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		107,209,558