Acquisition (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 02, 2019	Jan. 01, 2019
Business Acquisition [Line Items]
Purchase of business	$ 48,581	$ 0	$ 55,049
Business combination, contingent consideration, liability	30,896	3,070
Amortization of Intangible Assets	13,100	5,000	6,100
Revenues	1,049,443	966,916	658,781
Net income (loss) attributable to Cowen Inc.	24,631	42,819	(60,882)
Operating lease right-of-use assets	92,852	0			$ 103,694
Business Combination, Bargain Purchase, Gain Recognized, Amount	0	0	6,914
Restructuring Costs	0	$ 0	$ 8,763
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	8,900
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed	1,200
Revenues	34,800
Net income (loss) attributable to Cowen Inc.	$ 5,700
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