Regulatory Requirements (Details)	12 Months Ended
	Dec. 31, 2017 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	1,000,000
Net capital	40,200,000
Excess capital	39,200,000
Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and Securities Segregated under Securities Exchange Commission Regulation	20,300,000
Net capital requirement under alternative method	2,700,000
Required Net Capital under Commodity Exchange Act	108,700,000
Net capital	111,400,000
Excess capital	108,700,000
ATM Execution LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	300,000
Net capital	1,000,000
Excess capital	700,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	300,000
Net capital	12,300,000
Excess capital	12,000,000
Westminster Research
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	300,000
Net capital	7,600,000
Excess capital	7,300,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum Net Capital Required	300,000.0
Net capital	28,700,000
Options Clearing Corporation [Member] | Cowen Execution
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 | Ramius U.K., Ltd.
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	$ 200,000
Financial resources requirement	100,000
Excess financial resources	100,000
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	11,500,000
Financial resources requirement	7,100,000
Excess financial resources	4,400,000
U.K. Financial Services Authority | Cowen Execution Services Ltd
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	3,800,000
Financial resources requirement	2,600,000
Excess financial resources	1,200,000
Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and Securities Segregated under Securities Exchange Commission Regulation	10,500,000
Required Net Capital under Commodity Exchange Act	45,000
Special Reserve Accounts [Member] | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and Securities Segregated under Securities Exchange Commission Regulation	29,700,000
Special Reserve Accounts [Member] | Minimum | Cowen Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Cash and Securities Segregated under Securities Exchange Commission Regulation	$ 17,100,000