Regulatory Requirements (Details)	Sep. 30, 2016 USD ($)
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	$ 1,000,000
Net capital	78,300,000
Excess capital	77,300,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	11,700,000
Excess capital	11,400,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000.0
Net capital	22,100,000
ATM Execution LLC [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	2,700,000
Excess capital	2,400,000
U.K. Financial Services Authority | Ramius U.K., Ltd.
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	240,000
Financial resources requirement	60,000
Excess financial resources	180,000
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	3,900,000
Financial resources requirement	1,900,000
Excess financial resources	$ 2,000,000