Acquisition (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 02, 2019	Jan. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Purchase of business			$ 48,581	$ 0
Revenues	$ 252,047	$ 221,028	768,308	706,985
Net income (loss) attributable to Cowen Inc.	3,813	$ 15,488	19,408	$ 37,740
Operating lease right-of-use assets	96,664		96,664			$ 103,694	$ 0
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		6,700
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed	100		1,200
Revenues	14,100		26,300
Net income (loss) attributable to Cowen Inc.	$ 3,200		$ 5,600
Operating lease right-of-use assets					$ 3,200
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