Regulatory Requirements (Details)	6 Months Ended
	Jun. 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	6,392,000
Minimum net capital required	1,000,000.0
Net capital	368,111,000
Excess capital	361,719,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	5,170,000
Excess capital	4,920,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	22,057,000
Excess capital	21,807,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	24,603,000
Excess capital	24,353,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	2,171,000
Financial resources requirement	386,000
Excess financial resources	1,785,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000
Net capital	6,600,000
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	$ 39,959,000
Financial resources requirement	30,037,000
Excess financial resources	9,922,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	13,254,000
Financial resources requirement	3,414,000
Excess financial resources	9,840,000
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	30,700,000
Special Reserve Accounts | Minimum | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	14,000,000.0
PAB Reserve Bank Accounts | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	23,200,000
PAB Reserve Bank Accounts | Minimum | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 14,900,000