Acquisition (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 02, 2019
Business Acquisition [Line Items]
Purchase of business			$ 0	$ 48,581,000
Revenues	$ 387,735,000	$ 252,047,000	1,120,384,000	768,308,000
Net income (loss) attributable to Cowen Inc.	20,284,000	3,813,000	124,196,000	19,408,000
Quarton
Business Acquisition [Line Items]
Total estimated purchase price				103,000,000.0
Purchase of business				75,300,000
Business combination, contingent consideration, liability						$ 40,000,000.0
Other payments to acquire businesses				600,000
Business combination, contingency liability, retention of employee, bonus pool, maximum				$ 10,000,000.0
Business combination, contingency liability, retention of employee, bonus pool, period				5 years
Business combination, contingency liability, retention of employee, bonus pool, annual maximum payout				$ 2,500,000
Intangible assets						22,200,000
Amortization of Intangible Assets	$ 2,200,000	6,700,000	$ 2,200,000	6,700,000
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed		$ 100,000		$ 1,200,000
Common Stock Class A | Quarton
Business Acquisition [Line Items]
Business acquisition, equity interest issued or issuable, number of shares				1,033,350
Business acquisition, share price					$ 14.52
Fair Value, Measurements, Recurring | Level 3 | Quarton
Business Acquisition [Line Items]
Business combination, contingent consideration, liability						27,700,000
Minimum | Quarton
Business Acquisition [Line Items]
Business combination, contingent consideration, liability						10,000,000
Maximum | Quarton
Business Acquisition [Line Items]
Business combination, contingent consideration, liability						$ 40,000,000.0