Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2012	Mar. 06, 2013	Jun. 30, 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	Dec. 31, 2012
Entity Filer Category	Accelerated Filer
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Entity Common Stock, Shares Outstanding		112,512,598
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 234,568,972