Regulatory Requirements (Details)	9 Months Ended
	Sep. 30, 2021 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	8,854,000
Minimum net capital required	1,000,000
Net capital	324,607,000
Excess capital	315,753,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	5,910,000
Excess capital	5,660,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	24,487,000
Excess capital	24,237,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	2,299,000
Financial resources requirement	385,000
Excess financial resources	1,914,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000
Net capital	6,700,000
Options Clearing Corporation | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	$ 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	$ 38,928,000
Financial resources requirement	27,853,000
Excess financial resources	11,075,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	12,912,000
Financial resources requirement	3,734,000
Excess financial resources	9,178,000
Minimum | Cowen and Company
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	55,800,000
PAB Reserve Bank Accounts | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 14,200,000