Document and Entity Information	6 Months Ended
	Jun. 30, 2014	Aug. 06, 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	Jun. 30, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		115,010,131