Acquisition (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 02, 2019
Business Acquisition [Line Items]
Purchase of business	$ 0	$ 48,581
Revenues	313,809	224,097
Net income (loss) attributable to Cowen Inc.	(9,923)	9,783
Quarton
Business Acquisition [Line Items]
Total estimated purchase price		103,000
Purchase of business		75,300
Business combination, contingent consideration, liability				$ 40,000
Other payments to acquire businesses		600
Business combination, contingency liability, retention of employee, bonus pool, maximum		$ 10,000
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
Intangible assets				22,200
Amortization of Intangible Assets	$ 2,200	2,200
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed		1,100
Revenues		5,300
Net income (loss) attributable to Cowen Inc.		$ 1,600
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
Minimum | Quarton
Business Acquisition [Line Items]
Business combination, contingent consideration, liability				10,000
Maximum | Quarton
Business Acquisition [Line Items]
Business combination, contingent consideration, liability				$ 40,000