Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) cowenfund	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) cowenfund	Sep. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Operating right of use assets	$ 96,664		$ 96,664			$ 103,694	$ 0
Operating lease liabilities	100,934		100,934			$ 110,505	0
Variable Interest Entity, Consolidated, Carrying Amount, Assets	656,400		656,400				468,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	34,300		34,300				40,500
Cumulative Effect on Retained Earnings, Net of Tax				$ 559
Fees receivable	(101,508)		(101,508)				(111,946)
Other investments	204,168		204,168				181,407
Total Stockholders' Equity	817,055	$ 808,244	817,055	808,244	$ 817,424		$ 794,407	$ 793,207	$ 748,019
Investment banking	77,292	81,923	272,103	264,737
Revenues	252,047	221,028	768,308	706,985
Underwriting expenses	2,846	4,412	12,383	13,260
Costs and Expenses	219,964	212,329	689,984	663,566
Net gains (losses) on securities, derivatives and other investments	18,446	32,040	61,440	64,728
Net Income (Loss) Available to Common Stockholders, Basic	2,115	13,790	14,314	32,646
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	6,583	19,598	26,596	75,538
Gain (Loss) on Investments (excluding broker dealer)			(57,808)	(61,927)
Fees receivable, net of allowance			17,707	(6,211)
Payments to Acquire Other Investments			(15,899)	(21,355)
Revenue from Contract with Customer, Excluding Assessed Tax	175,943	182,337	583,682	584,940
Management Fees	7,300	7,546	21,480	22,336
Incentive income	$ 701	33	$ 724	97
Other investment companies
Number of funds, Consolidated | cowenfund	5		5
Furniture and Fixtures
Property, Plant and Equipment, Useful Life			5 years
Assets Held under Capital Leases
Property, Plant and Equipment, Useful Life			5 years
Minimum
Redeemable Noncontrolling Interest, Redemption Fee, Percent			1.00%
Minimum | Telephone and computer equipment
Property, Plant and Equipment, Useful Life			3 years
Minimum | Proprietary software
Property, Plant and Equipment, Useful Life			3 years
Maximum
Redeemable Noncontrolling Interest, Redemption Fee, Percent			5.00%
Maximum | Telephone and computer equipment
Property, Plant and Equipment, Useful Life			5 years
Maximum | Proprietary software
Property, Plant and Equipment, Useful Life			8 years
Hedge Funds
Asset Management Fees, Percent Fee			2.00%
Real Estate Funds | Minimum
Asset Management Fees, Percent Fee			0.25%
Real Estate Funds | Maximum
Asset Management Fees, Percent Fee			1.50%
Other Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee			0.20%
Other Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee			1.00%
During investment period | Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee			1.00%
During investment period | Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee			2.00%
After investment period | Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee			0.50%
After investment period | Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee			2.00%
Hedge Funds
Incentive Fees, Percent Fee			20.00%
Operating Company
Revenue from Contract with Customer, Excluding Assessed Tax	$ 175,695	181,386	$ 582,680	582,160
Management Fees	7,052	6,595	20,492	19,556
Incentive income	701	33	710	97
Operating Company | Underwriting fees
Revenue from Contract with Customer, Excluding Assessed Tax	38,936	46,537	162,140	181,056
Operating Company | Strategic/financial advisory fees
Revenue from Contract with Customer, Excluding Assessed Tax	17,889	22,195	49,565	51,053
Operating Company | Placement and sales agent fees
Revenue from Contract with Customer, Excluding Assessed Tax	15,842	13,191	48,855	32,628
Operating Company | Expense reimbursements from clients
Revenue from Contract with Customer, Excluding Assessed Tax	4,625	0	11,543	0
Operating Company | Investment Banking Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	77,292	81,923	272,103	264,737
Operating Company | Commissions
Revenue from Contract with Customer, Excluding Assessed Tax	82,715	84,882	263,831	270,917
Operating Company | Trade conversion
Revenue from Contract with Customer, Excluding Assessed Tax	3,700	2,931	12,364	12,268
Operating Company | Equity and credit research fees
Revenue from Contract with Customer, Excluding Assessed Tax	4,235	5,022	13,180	14,585
Operating Company | Brokerage
Revenue from Contract with Customer, Excluding Assessed Tax	90,650	92,835	289,375	297,770
Asset Company
Revenue from Contract with Customer, Excluding Assessed Tax	248	951	1,002	2,780
Management Fees	248	951	988	2,780
Incentive income	0	0	14	0
Operating Segments
Investment banking	70,022	77,510	257,234	251,476
Revenues	217,093	226,233	694,953	702,050
Costs and Expenses	202,040	196,188	630,192	613,561
Net Income (Loss) Available to Common Stockholders, Basic	5,547	20,986	36,306	63,390
Management Fees	10,907	12,386	31,804	37,998
Incentive income	14,377	6,910	35,353	21,470
Operating Segments | Operating Company
Investment banking	70,022	77,510	257,234	251,476
Revenues	216,686	217,664	687,121	693,026
Costs and Expenses	200,637	189,516	623,167	601,480
Net Income (Loss) Available to Common Stockholders, Basic	8,289	21,082	40,581	72,331
Management Fees	10,321	10,840	30,016	33,057
Incentive income	15,384	1,859	34,654	17,219
Operating Segments | Asset Company
Investment banking	0	0	0	0
Revenues	407	8,569	7,832	9,024
Costs and Expenses	1,403	6,672	7,025	12,081
Net Income (Loss) Available to Common Stockholders, Basic	(2,742)	(96)	(4,275)	(8,941)
Management Fees	586	1,546	1,788	4,941
Incentive income	$ (1,007)	$ 5,051	$ 699	$ 4,251