Regulatory Requirements (Details)	Dec. 31, 2016 USD ($)
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	$ 1,000,000
Net capital	65,300,000
Excess capital	64,300,000
Cowen Prime
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	13,900,000
Excess capital	13,600,000
RCG Insurance Company
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	300,000.0
Net capital	22,500,000
ATM Execution LLC [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	3,500,000
Excess capital	3,200,000
U.K. Financial Services Authority | Ramius U.K., Ltd.
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	220,000
Financial resources requirement	50,000
Excess financial resources	170,000
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	9,600,000
Financial resources requirement	2,600,000
Excess financial resources	$ 7,000,000