Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 05, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	Cowen Inc.
Entity Central Index Key	0001466538
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		28,985,051
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 476,392,881