Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) cowenfund	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) cowenfund	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Total net assets of consolidated VIEs	$ 254,600		$ 254,600		$ 482,800
Deferred rent	9,100		9,100		12,800
Cumulative Effect on Retained Earnings, Net of Tax	600		559
Fees receivable	111,756		111,756		111,784
Other investments	167,428		167,428		141,548
Total Stockholders' Equity	808,244		808,244		$ 748,019
Investment banking	81,923	$ 57,383	264,737	$ 158,082
Incentive income	33	1,945	97	6,217
Revenues	221,028	178,830	706,985	454,331
Underwriting expenses	4,412	0	13,260	0
Costs and Expenses	211,149	179,216	660,311	487,456
Net gains (losses) on securities, derivatives and other investments	32,040	18,326	64,728	63,101
Net Income (Loss) Available to Common Stockholders, Basic	13,790	2,968	32,646	9,996
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 20,778	$ 9,828	78,793	50,502
Gain (Loss) on Investments (excluding broker dealer)			(61,927)	(76,325)
Fees receivable, net of allowance			(6,211)	(28,839)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations			295,986
Payments to Acquire Other Investments			(21,355)	$ (72,834)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations			$ (10,925)
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 | Telephone and computer equipment
Property, Plant and Equipment, Useful Life			3 years
Minimum | Computer Software, Intangible Asset
Property, Plant and Equipment, Useful Life			3 years
Minimum | Aircraft
Property, Plant and Equipment, Useful Life			10 years
Minimum | Flight Equipment
Property, Plant and Equipment, Useful Life			4 years
Maximum | Telephone and computer equipment
Property, Plant and Equipment, Useful Life			5 years
Maximum | Computer Software, Intangible Asset
Property, Plant and Equipment, Useful Life			8 years
Maximum | Aircraft
Property, Plant and Equipment, Useful Life			20 years
Maximum | Flight Equipment
Property, Plant and Equipment, Useful Life			10 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%
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
Fees receivable	$ 123,495		$ 123,495
Other investments	155,689		155,689
Total Stockholders' Equity	811,737		811,737
Investment banking	77,511		251,477
Incentive income	7,809		11,283
Revenues	224,392		704,911
Underwriting expenses	0		0
Costs and Expenses	206,737		647,051
Net gains (losses) on securities, derivatives and other investments	38,862		72,980
Net Income (Loss) Available to Common Stockholders, Basic	14,744		35,580
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest			81,727
Gain (Loss) on Investments (excluding broker dealer)			55,852
Fees receivable, net of allowance			14,408
Net Cash Provided by (Used in) Operating Activities, Continuing Operations			296,798
Payments to Acquire Other Investments			(22,167)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations			(11,737)
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Fees receivable	11,739		11,739
Other investments	(11,739)		(11,739)
Total Stockholders' Equity	3,493		3,493
Investment banking	(4,412)		(13,260)
Incentive income	7,776		11,186
Revenues	3,364		(2,074)
Underwriting expenses	(4,412)		(13,260)
Costs and Expenses	(4,412)		(13,260)
Net gains (losses) on securities, derivatives and other investments	6,822		8,252
Net Income (Loss) Available to Common Stockholders, Basic	$ 954		2,934
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest			2,934
Gain (Loss) on Investments (excluding broker dealer)			(6,075)
Fees receivable, net of allowance			8,197
Net Cash Provided by (Used in) Operating Activities, Continuing Operations			812
Payments to Acquire Other Investments			812
Net Cash Provided by (Used in) Investing Activities, Continuing Operations			$ (812)