Significant Accounting Policies - Annual (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) cowenfund	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)
Organization [Line Items]
Operating lease right-of-use assets	$ 78,241	$ 92,852		$ 103,700
Operating lease liabilities	82,735	97,581		$ 110,500
Revenue from Contract with Customer, Excluding Assessed Tax	1,338,501	797,386	$ 793,332
Fees receivable	160,349	126,358
Other investments	255,027	185,722
Total Stockholders' Equity	969,497	809,855
Management Fees	47,515	32,608	29,658
Investment banking	769,486	375,025	357,222
Incentive income	592	1,547	3,117
Revenues	1,623,304	1,049,443	966,916
Underwriting expenses	22,565	15,067	15,282
Professional, advisory and other fees	59,990	48,385	40,957
Service fees	25,378	23,707	20,198
Communications	32,593	31,894	30,801
Occupancy and equipment	36,559	39,726	41,602
Client services and business development	20,526	50,371	35,927
Other expenses	33,195	23,114	22,014
Costs and Expenses	1,292,362	948,864	890,944
Net gains (losses) on securities, derivatives and other investments	175,877	80,409	68,043
Net income (loss) attributable to Cowen Inc. common stockholders	209,571	17,839	36,027
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	207,064	55,870	79,879
Gain (Loss) on Investments (excluding broker dealer)	161,699	74,284	54,032
Increase (Decrease) in Fees Receivable	33,991	7,143	6,401
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	521,279	(179,910)	324,176
Payments to Acquire Other Investments	48,634	19,812	28,092
Net Cash Provided by (Used in) Investing Activities	(20,822)	(47,627)	(17,882)
Liabilities	6,659,078	3,866,575
Assets	7,828,199	5,162,025
Variable Interest Entity, Primary Beneficiary [Member]
Organization [Line Items]
Liabilities	10,100	24,900
Assets	325,500	685,400
Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,337,542	796,123	789,754
Management Fees	46,556	31,361	26,080
Incentive income	592	1,532	3,117
Asset Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	959	1,263	3,578
Management Fees	959	1,248	3,578
Incentive income	$ 0	15	0
Telephone and computer equipment | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Telephone and computer equipment | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Proprietary software | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Proprietary software | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	4 years
Furniture and Fixtures
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Hedge Funds | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
During investment period | Minimum | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
During investment period | Maximum | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
After investment period | Healthcare Royalty Partners | Minimum | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
After investment period | Healthcare Royalty Partners | Maximum | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
After investment period | Other Healthcare Royalty Partners | Minimum | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	0.20%
After investment period | Other Healthcare Royalty Partners | Maximum | Operating Company
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
Other investment companies
Organization [Line Items]
Number of funds, Consolidated | cowenfund	3
Underwriting fees | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 427,670	211,666	215,723
Strategic/financial advisory fees | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	190,958	79,208	81,733
Placement and sales agent fees | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	133,171	69,070	46,888
Expense reimbursements from clients | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	17,687	15,081	12,878
Investment banking revenue | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	769,486	375,025	357,222
Brokerage | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	520,908	388,205	403,335
Commissions | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	485,398	356,668	366,090
Trade conversion | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	15,753	12,531	17,061
Equity and credit research fees | Operating Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 19,757	$ 19,006	$ 20,184