Regulatory Requirements (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Apr. 02, 2020
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	$ 250,000
Interest Income, Related Party	100,000
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	1,000,000.0
Net capital	134,102,000
Excess capital	133,102,000
Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	7,066,000
Required net capital under commodity exchange act	107,300,000
Cash and securities segregated under securities exchange commission regulation	19,000,000.0
Net capital	114,384,000
Excess capital	107,318,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	873,000
Financial resources requirement	387,000
Excess financial resources	486,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	4,821,000
Excess capital	4,571,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	18,122,000
Excess capital	17,872,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000
Net capital	18,856,000
Excess capital	18,606,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000
Net capital	31,800,000
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	$ 16,378,000
Financial resources requirement	6,677,000
Excess financial resources	9,701,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	11,609,000
Financial resources requirement	2,963,000
Excess financial resources	8,646,000
Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	10,500,000
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	36,000,000.0	$ 11,500,000
Special Reserve Accounts | Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 38,900,000