Regulatory Requirements (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Regulatory Requirements for Broker-Dealers [Line Items]
Interest Income, Related Party	$ 100,000	$ 0
Minimum net capital required	250,000	250,000
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	1,000,000	1,000,000
Net capital	88,162,000	88,162,000
Excess capital	87,162,000	87,162,000
Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	17,000,000	17,000,000
Net capital requirement under alternative method	1,069,000	1,069,000
Required net capital under commodity exchange act	103,500,000	103,500,000
Net capital	104,559,000	104,559,000
Excess capital	103,490,000	103,490,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	1,088,000	1,088,000
Financial resources requirement	383,000	383,000
Excess financial resources	705,000	705,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000	250,000
Net capital	5,538,000	5,538,000
Excess capital	5,288,000	5,288,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000	250,000
Net capital	17,449,000	17,449,000
Excess capital	17,199,000	17,199,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000	250,000
Net capital	24,048,000	24,048,000
Excess capital	23,798,000	23,798,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000.0	300,000.0
Net capital	32,900,000	32,900,000
Options Clearing Corporation | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	2,000,000	$ 2,000,000
Minimum net capital required, percent		2.00%
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	14,475,000	$ 14,475,000
Financial resources requirement	9,925,000	9,925,000
Excess financial resources	4,550,000	4,550,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	4,957,000	4,957,000
Financial resources requirement	2,043,000	2,043,000
Excess financial resources	2,914,000	2,914,000
Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	6,300,000	6,300,000
Required net capital under commodity exchange act	45,000	45,000
Special Reserve Accounts | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	26,900,000	26,900,000
Special Reserve Accounts | Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 20,300,000	$ 20,300,000