Document and Entity Information Document (USD $)	12 Months Ended
	Dec. 31, 2011	Mar. 08, 2012	Jun. 30, 2011
Entity Registrant Name	Cowen Group, Inc.
Entity Central Index Key	0001466538
Current Fiscal Year End Date	--12-31
Current Fiscal Year End Date	Dec. 31, 2011
Entity Filer Category	Accelerated Filer
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Entity Common Stock, Shares Outstanding		114,377,005
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 301,319,256