Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 28, 2021
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	001-34516
Entity Registrant Name	Cowen Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-0423711
Entity Address, Address Line One	599 Lexington Avenue
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10022
City Area Code	646
Local Phone Number	562-1010
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		27,287,075
Entity Central Index Key	0001466538
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false
Common Stock Class A | The Nasdaq Global Market
Entity Information [Line Items]
Title of 12(b) Security	Class A Common Stock, par value $0.01 per share
Trading Symbol	COWN
Security Exchange Name	NASDAQ
2033 Notes | The Nasdaq Global Market
Entity Information [Line Items]
Title of 12(b) Security	7.75% Senior Notes due 2033
Trading Symbol	COWNL
Security Exchange Name	NASDAQ