Significant Accounting Policies - Annual (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) cowenfund	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) cowenfund	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 793,332
Management Fees									29,658	$ 33,245	$ 40,612
Cumulative Effect on Retained Earnings, Net of Tax									(559)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 4,424	$ 20,778	$ 30,006	$ 28,009	$ (87,593)	$ 9,828	$ 28,585	$ 12,089	83,217	(37,091)	(12,395)
Investment banking									357,222	223,614	133,279
Brokerage									413,582	293,610	199,180
Accrued Fees and Other Revenue Receivable	111,946				111,784				111,946	111,784
Total net assets of consolidated VIEs	427,500				482,800				427,500	482,800
Deferred rent	8,242				12,822				8,242	12,822
Other investments	181,407				141,548				181,407	141,548
Total Stockholders' Equity	794,407				748,019				794,407	748,019
Incentive income									3,117	5,383	8,334
Revenues	259,931	221,028	234,573	251,384	204,450	178,830	160,530	114,971	966,916	658,781	471,565
Underwriting expenses									15,282	0	0
Professional, advisory and other fees									40,957	31,942	23,190
Service fees									20,198	14,999	7,918
Communications									30,801	23,460	17,768
Occupancy and equipment									41,602	35,184	32,286
Client services and business development									35,927	28,327	27,828
Other expenses									22,014	18,196	14,815
Costs and Expenses									887,606	696,649	517,810
Net gains (losses) on securities, derivatives and other investments									68,043	76,179	23,381
Net income (loss) attributable to Cowen Inc. common stockholders	$ 3,381	$ 13,790	$ 3,701	$ 15,155	$ (77,670)	$ 2,968	$ 5,742	$ 1,286	36,027	(67,674)	(26,069)
Gain (Loss) on Investments (excluding broker dealer)									54,032	78,303	28,878
Increase (Decrease) in Fees Receivable									6,401	29,669	12,632
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									324,536	3,398	(351,517)
Payments to Acquire Other Investments									28,092	10,419	33,786
Net Cash Provided by (Used in) Investing Activities									$ (17,882)	$ (2,397)	$ 58,514
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 | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									3 years
Telephone and computer equipment | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									5 years
Computer Software, Intangible Asset | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									3 years
Computer Software, Intangible Asset | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									8 years
Furniture and Fixtures
Organization [Line Items]
Property, Plant and Equipment, Useful Life									5 years
Assets Held under Capital Leases
Organization [Line Items]
Property, Plant and Equipment, Useful Life									5 years
Aircraft | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									10 years
Aircraft | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									20 years
Flight Equipment | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									4 years
Flight Equipment | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life									10 years
Other investment companies
Organization [Line Items]
Number of funds, Consolidated | cowenfund	4								4
Accounting Standards Update 2014-09
Organization [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									$ 83,659
Investment banking									329,062
Accrued Fees and Other Revenue Receivable	$ 129,941								129,941
Other investments	163,412								163,412
Total Stockholders' Equity	795,409								795,409
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									859,446
Net gains (losses) on securities, derivatives and other investments									83,512
Net income (loss) attributable to Cowen Inc. common stockholders									36,469
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									325,347
Payments to Acquire Other Investments									28,904
Net Cash Provided by (Used in) Investing Activities									(18,694)
Investment Bank
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									357,222
Underwriting fees
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									215,724
Strategic/financial advisory fees
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									81,733
Placement and sales agent fees
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									46,887
Expense reimbursements from clients
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									12,878
Brokerage
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									403,335
Commissions
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									366,090
Trade conversion
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									17,061
Equity and credit research fees
Organization [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									20,184
Adjustment | Accounting Standards Update 2014-09
Organization [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									442
Investment banking									(28,160)
Accrued Fees and Other Revenue Receivable	17,995								17,995
Other investments	(17,995)								(17,995)
Total Stockholders' Equity	$ 1,002								1,002
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
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)