Regulatory Requirements (Details) (USD $)	Sep. 30, 2014
Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	$ 1,000,000
Net capital	109,400,000
Excess capital	108,400,000
ATM Execution
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	2,100,000
Excess capital	1,900,000
ATM USA
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	900,000
Excess capital	700,000
Cowen Equity Finance
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	14,500,000
Excess capital	14,300,000
U.K. Financial Services Authority | Raimus U.K., Ltd.
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	340,000
Financial resources requirement	110,000
Excess financial resources	230,000
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	3,400,000
Financial resources requirement	2,100,000
Excess financial resources	1,300,000
H.K. Securities and Futures Commission | Cowen and Company (Asia) Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	700,000
Financial resources requirement	400,000
Excess financial resources	$ 300,000