Significant Accounting Policies - Annual (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) cowenfund	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Organization [Line Items]
Operating right of use assets	$ 92,852	$ 0		$ 103,694
Operating lease liabilities	97,581	0		$ 110,505
Revenue from Contract with Customer, Excluding Assessed Tax	797,386	793,332
Accrued Fees and Other Revenue Receivable	126,358	111,946
Other investments	185,722	181,407
Total Stockholders' Equity	809,855	794,407
Management Fees	32,608	29,658	$ 33,245
Investment banking	375,025	357,222	223,614
Incentive income	1,547	3,117	5,383
Revenues	1,049,443	966,916	658,781
Underwriting expenses	15,067	15,282	0
Professional, advisory and other fees	48,385	40,957	31,942
Service fees	23,707	20,198	14,999
Communications	31,894	30,801	23,460
Occupancy and equipment	39,726	41,602	35,184
Client services and business development	50,371	35,927	28,327
Other expenses	23,114	22,014	18,196
Costs and Expenses	948,864	890,944	696,649
Net gains (losses) on securities, derivatives and other investments	80,409	68,043	76,179
Net income (loss) attributable to Cowen Inc. common stockholders	17,839	36,027	(67,674)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	55,870	79,879	(37,091)
Gain (Loss) on Investments (excluding broker dealer)	74,284	54,032	78,303
Increase (Decrease) in Fees Receivable	7,143	6,401	29,669
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(179,910)	324,176	3,398
Payments to Acquire Other Investments	19,812	28,092	10,419
Net Cash Provided by (Used in) Investing Activities	(47,627)	(17,882)	$ (2,397)
Liabilities	3,866,575	2,270,093
Assets	5,162,025	3,346,303
Variable Interest Entity, Primary Beneficiary [Member]
Organization [Line Items]
Liabilities	24,900	40,500
Assets	685,400	468,000
Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	796,123	789,754
Management Fees	31,361	26,080
Incentive income	1,532	3,117
Asset Company
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,263	3,578
Management Fees	1,248	3,578
Incentive income	$ 15	0
Minimum
Organization [Line Items]
Redeemable Noncontrolling Interest, Redemption Fee, Percent	1.00%
Maximum
Organization [Line Items]
Redeemable Noncontrolling Interest, Redemption Fee, Percent	5.00%
Telephone and computer equipment
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Proprietary software
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
Real Estate Funds | Minimum | Asset Company
Organization [Line Items]
Asset Management Fees, Percent Fee	0.25%
Real Estate Funds | Maximum | Asset Company
Organization [Line Items]
Asset Management Fees, Percent Fee	1.50%
Hedge Funds | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
Hedge Funds | Asset Company
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
During investment period | Healthcare Royalty Partners | Minimum | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
During investment period | Healthcare Royalty Partners | Maximum | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
After investment period | Healthcare Royalty Partners | Minimum | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	0.50%
After investment period | Healthcare Royalty Partners | Maximum | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
After investment period | Other Healthcare Royalty Partners | Minimum | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	0.20%
After investment period | Other Healthcare Royalty Partners | Maximum | Operating Company [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
Other investment companies
Organization [Line Items]
Number of funds, Consolidated | cowenfund	5
Accounting Standards Update 2014-09
Organization [Line Items]
Accrued Fees and Other Revenue Receivable		129,941
Other investments		163,412
Total Stockholders' Equity		795,409
Investment banking		329,062
Incentive income		19,028
Revenues		954,667
Underwriting expenses		0
Professional, advisory and other fees		33,096
Service fees		20,156
Communications		29,352
Occupancy and equipment		41,588
Client services and business development		32,554
Other expenses		21,875
Costs and Expenses		862,784
Net gains (losses) on securities, derivatives and other investments		83,512
Net income (loss) attributable to Cowen Inc. common stockholders		36,469
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		80,321
Gain (Loss) on Investments (excluding broker dealer)		41,701
Increase (Decrease) in Fees Receivable		18,363
Net Cash Provided by (Used in) Operating Activities, Continuing Operations		324,987
Payments to Acquire Other Investments		28,904
Net Cash Provided by (Used in) Investing Activities		(18,694)
Underwriting fees | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 211,666	215,723
Strategic/financial advisory fees | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	79,208	81,733
Placement and sales agent fees | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	69,070	46,888
Expense reimbursements from clients | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	15,081	12,878
Investment banking revenue | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	375,025	357,222
Brokerage | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	388,205	403,335
Commissions | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	356,668	366,090
Trade conversion | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	12,531	17,061
Equity and credit research fees | Operating Company [Member]
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 19,006	20,184
Adjustment | Accounting Standards Update 2014-09
Organization [Line Items]
Accrued Fees and Other Revenue Receivable		17,995
Other investments		(17,995)
Total Stockholders' Equity		1,002
Investment banking		(28,160)
Incentive income		15,911
Revenues		(12,249)
Underwriting expenses		(15,282)
Professional, advisory and other fees		(7,861)
Service fees		(42)
Communications		(1,449)
Occupancy and equipment		(14)
Client services and business development		(3,373)
Other expenses		(139)
Costs and Expenses		(28,160)
Net gains (losses) on securities, derivatives and other investments		15,469
Net income (loss) attributable to Cowen Inc. common stockholders		442
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		442
Gain (Loss) on Investments (excluding broker dealer)		(12,331)
Increase (Decrease) in Fees Receivable		11,962
Net Cash Provided by (Used in) Operating Activities, Continuing Operations		811
Payments to Acquire Other Investments		812
Net Cash Provided by (Used in) Investing Activities		$ (812)