Regulatory Requirements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2017
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	$ 250,000
Interest Income, Related Party	100,000	$ 100,000
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	1,000,000.0
Net capital	91,348,000
Excess capital	90,348,000
Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	2,673,000
Required net capital under commodity exchange act	103,200,000
Cash and securities segregated under securities exchange commission regulation	17,600,000
Net capital	105,822,000
Excess capital	103,149,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	1,307,000
Financial resources requirement	385,000
Excess financial resources	922,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	5,354,000
Excess capital	5,104,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	13,659,000
Excess capital	13,409,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	14,797,000
Excess capital	14,547,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000
Net capital	32,800,000
Options Clearing Corporation | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	$ 2,000,000.0
Minimum net capital required, percent	2.00%
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	$ 15,988,000
Financial resources requirement	7,597,000
Excess financial resources	8,391,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	11,808,000
Financial resources requirement	2,509,000
Excess financial resources	9,299,000
Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	5,500,000
Minimum | Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Required net capital under commodity exchange act	45,000
Special Reserve Accounts | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	22,500,000
Special Reserve Accounts | Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 14,800,000