| Document and Entity Information | Jun. 24, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jun. 24, 2021 |
| Entity Registrant Name | COWEN INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-34516 |
| 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 | 212 |
| Local Phone Number | 845-7900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001466538 |
| Amendment Flag | false |
| Class A Common Stock, par value $0.01 per share | 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 |
| 7.75% Senior Notes due 2033 | 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 |
