Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) cowenfund	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) cowenfund	Jun. 30, 2018 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Operating right of use assets	$ 101,628		$ 101,628		$ 103,694	$ 0
Operating Lease, Liability	106,873		106,873		$ 110,505	0
Variable Interest Entity, Consolidated, Carrying Amount, Assets	606,200		606,200			468,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	11,200		11,200			40,500
Cumulative Effect on Retained Earnings, Net of Tax				$ 559
Fees receivable	(126,500)		(126,500)			(111,946)
Other investments	200,297		200,297			181,407
Total Stockholders' Equity	817,424		817,424			$ 794,407
Investment banking	114,705	$ 84,826	194,811	182,814
Revenues	292,164	234,573	516,261	485,957
Underwriting expenses	6,406	4,785	9,537	8,848
Costs and Expenses	247,112	223,416	467,901	449,162
Net gains (losses) on securities, derivatives and other investments	3,910	16,719	42,994	32,688
Net Income (Loss) Available to Common Stockholders, Basic	4,114	3,701	12,199	18,856
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	10,143	30,006	22,132	58,015
Gain (Loss) on Investments (excluding broker dealer)			(42,314)	(28,237)
Fees receivable, net of allowance			(7,285)	(32,917)
Payments to Acquire Other Investments			(9,513)	(20,227)
Revenue from Contract with Customer, Excluding Assessed Tax	228,007	194,794	407,740	401,871
Management Fees	7,039	7,373	14,180	14,790
Incentive income	$ 8	48	$ 23	64
Other investment companies
Number of funds, Consolidated | cowenfund	4		4
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 | Maximum
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	$ 227,764	193,708	$ 406,984	400,041
Management Fees	6,796	6,287	13,439	12,960
Incentive income	8	48	8	64
Operating Company | Underwriting fees
Revenue from Contract with Customer, Excluding Assessed Tax	75,169	62,751	123,204	134,519
Operating Company | Strategic/financial advisory fees
Revenue from Contract with Customer, Excluding Assessed Tax	11,131	14,498	31,676	28,858
Operating Company | Placement and sales agent fees
Revenue from Contract with Customer, Excluding Assessed Tax	23,515	7,577	33,013	19,437
Operating Company | Expense reimbursements from clients
Revenue from Contract with Customer, Excluding Assessed Tax	4,890	0	6,918	0
Operating Company | Investment Banking Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	114,705	84,826	194,811	182,814
Operating Company | Commissions
Revenue from Contract with Customer, Excluding Assessed Tax	96,341	91,008	181,116	185,303
Operating Company | Trade conversion
Revenue from Contract with Customer, Excluding Assessed Tax	4,559	5,815	8,665	9,337
Operating Company | Equity and credit research fees
Revenue from Contract with Customer, Excluding Assessed Tax	5,355	5,724	8,945	9,563
Operating Company | Brokerage
Revenue from Contract with Customer, Excluding Assessed Tax	106,255	102,547	198,726	204,203
Asset Company
Revenue from Contract with Customer, Excluding Assessed Tax	243	1,086	756	1,830
Management Fees	243	1,086	741	1,830
Incentive income	0	0	15	0
Operating Segments
Investment banking	104,216	80,042	187,212	173,966
Revenues	244,389	234,310	477,861	475,817
Costs and Expenses	218,819	205,965	426,004	415,298
Net Income (Loss) Available to Common Stockholders, Basic	15,483	20,043	30,760	42,404
Management Fees	10,468	12,486	20,898	25,613
Incentive income	4,229	9,363	20,976	14,559
Operating Segments | Operating Company
Investment banking	104,216	80,042	187,212	173,966
Revenues	238,652	234,152	470,434	475,363
Costs and Expenses	215,808	203,684	421,562	410,059
Net Income (Loss) Available to Common Stockholders, Basic	14,757	24,372	32,290	51,250
Management Fees	9,968	10,938	19,695	22,217
Incentive income	2,633	12,321	19,270	15,360
Operating Segments | Asset Company
Investment banking	0	0	0	0
Revenues	5,737	158	7,427	454
Costs and Expenses	3,011	2,281	4,442	5,239
Net Income (Loss) Available to Common Stockholders, Basic	726	(4,329)	(1,530)	(8,846)
Management Fees	500	1,548	1,203	3,396
Incentive income	$ 1,596	$ (2,958)	$ 1,706	$ (801)