Regulatory Requirements (Details)	9 Months Ended
	Sep. 30, 2020 USD ($)
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	$ 250,000
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	3,043,000
Minimum net capital required	1,000,000.0
Required net capital under commodity exchange act	385,600,000
Cash and securities segregated under securities exchange commission regulation	43,900,000
Net capital	388,606,000
Excess capital	385,563,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	1,394,000
Financial resources requirement	387,000
Excess financial resources	1,007,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	4,015,000
Excess capital	3,765,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	19,432,000
Excess capital	19,182,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	5,305,000
Excess capital	5,055,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000
Net capital	6,100,000
Options Clearing Corporation | Cowen and Company
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	$ 22,892,000
Financial resources requirement	17,882,000
Excess financial resources	5,010,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	11,501,000
Financial resources requirement	4,333,000
Excess financial resources	7,168,000
Minimum | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	20,500,000
Minimum | Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Required net capital under commodity exchange act	45,000
Special Reserve Accounts | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	35,200,000
Special Reserve Accounts | Minimum | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 16,600,000