Regulatory Requirements (Details) (USD $)	Dec. 31, 2014
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	$ 1,000,000
Net capital	42,900,000
Excess capital	41,900,000
ATM Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	3,100,000
Excess capital	2,800,000
ATM USA
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	1,100,000
Excess capital	800,000
Cowen Equity Finance
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	14,900,000
Excess capital	14,600,000
U.K. Financial Services Authority | Raimus U.K., Ltd.
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	320,000
Financial resources requirement	110,000
Excess financial resources	210,000
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	3,300,000
Financial resources requirement	2,200,000
Excess financial resources	1,100,000
H.K. Securities and Futures Commission | Cowen and Company (Asia) Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	500,000
Financial resources requirement	400,000
Excess financial resources	$ 100,000