Regulatory Requirements (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Regulatory Requirements for Broker-Dealers [Line Items]
Interest Income, Related Party	$ 0	$ 0
Minimum net capital required	250,000	250,000
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	1,000,000	1,000,000
Net capital	95,807,000	95,807,000
Excess capital	94,807,000	94,807,000
Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	15,600,000	15,600,000
Net capital requirement under alternative method	2,541,000	2,541,000
Required net capital under commodity exchange act	114,400,000	114,400,000
Net capital	116,967,000	116,967,000
Excess capital	114,426,000	114,426,000
Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	1,092,000	1,092,000
Financial resources requirement	384,000	384,000
Excess financial resources	708,000	708,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000	250,000
Net capital	5,375,000	5,375,000
Excess capital	5,125,000	5,125,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000	250,000
Net capital	12,091,000	12,091,000
Excess capital	11,841,000	11,841,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	250,000	250,000
Net capital	21,602,000	21,602,000
Excess capital	21,352,000	21,352,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000.0	300,000.0
Net capital	32,500,000	32,500,000
Options Clearing Corporation | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	2,000,000	$ 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	15,479,000	$ 15,479,000
Financial resources requirement	8,562,000	8,562,000
Excess financial resources	6,917,000	6,917,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	5,158,000	5,158,000
Financial resources requirement	3,087,000	3,087,000
Excess financial resources	2,071,000	2,071,000
Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Required net capital under commodity exchange act	45,000	45,000
Special Reserve Accounts | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	27,700,000	27,700,000
Special Reserve Accounts | Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and securities segregated under securities exchange commission regulation	$ 21,900,000	$ 21,900,000