Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) cowenfund	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Investment banking	$ 97,988	$ 36,553
Incentive income	16	546
Revenues	251,384	114,971
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	28,009	12,089
Fees receivable	136,541		$ 111,784
Other investments	157,293		141,548
Total net assets of consolidated VIEs	425,000		482,800
Deferred rent	11,900		12,800
Cumulative effect of the adoption of the new revenue recognition standard (see Note 3)	559
Gain (Loss) on Investments (excluding broker dealer)	(7,924)	(33,493)
Fees receivable, net of allowance	(30,996)	(4,042)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(35,506)	(53,354)
Payments to Acquire Other Investments	(11,475)	(4,934)
Underwriting expenses	4,063	0
Costs and Expenses	250,286	140,373
Net gains (losses) on securities, derivatives and other investments	15,969	26,056
Total Stockholders' Equity	758,787		$ 748,019
Net Income (Loss) Available to Common Stockholders, Basic	15,155	1,286
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	$ (5,395)	$ 12,296
Other investment companies
Number of funds, Consolidated | cowenfund	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 | 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 | Leasehold improvements
Property, Plant and Equipment, Useful Life	2 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 | Leasehold improvements
Property, Plant and Equipment, Useful Life	15 years
Maximum | Aircraft
Property, Plant and Equipment, Useful Life	20 years
Maximum | Flight Equipment
Property, Plant and Equipment, Useful Life	10 years
Hedge Funds [Member] | 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 [Member] | Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee	1.00%
During investment period [Member] | Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee	2.00%
After investment period [Member] | Healthcare Royalty Partners | Minimum
Asset Management Fees, Percent Fee	0.50%
After investment period [Member] | Healthcare Royalty Partners | Maximum
Asset Management Fees, Percent Fee	2.00%
Hedge Funds [Member]
Incentive Fees, Percent Fee	20.00%
Accounting Standards Update 2014-09 [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Investment banking	$ 93,925
Incentive income	2,323
Revenues	249,628
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	28,867
Fees receivable	142,940
Other investments	150,894
Gain (Loss) on Investments (excluding broker dealer)	(7,189)
Fees receivable, net of allowance	(31,777)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(34,694)
Payments to Acquire Other Investments	(12,287)
Underwriting expenses	0
Costs and Expenses	246,223
Net gains (losses) on securities, derivatives and other investments	17,423
Total Stockholders' Equity	760,199
Net Income (Loss) Available to Common Stockholders, Basic	16,008
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(6,207)
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Investment banking	(4,063)
Incentive income	2,307
Revenues	(1,756)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	858
Fees receivable	6,399
Other investments	(6,399)
Gain (Loss) on Investments (excluding broker dealer)	735
Fees receivable, net of allowance	(781)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	812
Payments to Acquire Other Investments	(812)
Underwriting expenses	(4,063)
Costs and Expenses	(4,063)
Net gains (losses) on securities, derivatives and other investments	1,454
Total Stockholders' Equity	1,412
Net Income (Loss) Available to Common Stockholders, Basic	853
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	$ (812)