Regulatory Requirements (Details) (USD $)	Mar. 31, 2013
U.S. Securities and Exchange Commission | Cowen and Company
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	$ 1,000,000
Net capital	32,700,000
Excess net capital under alternative method	31,700,000
U.S. Securities and Exchange Commission | Cowen and Company | Minimum
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	1,000,000.0
U.S. Securities and Exchange Commission | Cowen Capital, LLC
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	1,000,000.0
Net capital	3,500,000
Excess capital	2,500,000
U.S. Securities and Exchange Commission | Cowen Capital, LLC | Minimum
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	1,000,000.0
Net capital requirement as a percentage of indebtedness	6.667%
U.S. Securities and Exchange Commission | ATM USA
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	10,000
Net capital	520,000
Excess capital	510,000
U.S. Securities and Exchange Commission | ATM USA | Minimum
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	5,000
Net capital requirement as a percentage of indebtedness	6.667%
U.S. Securities and Exchange Commission | Cowen Equity Finance
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	250,000
Net capital	8,900,000
Excess capital	8,700,000
U.S. Securities and Exchange Commission | Cowen Securities
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	200,000
Net capital	1,900,000
Excess capital	1,700,000
U.S. Securities and Exchange Commission | Cowen Securities | Minimum
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	236,000
Net capital requirement as a percentage of indebtedness	6.667%
U.K. Financial Services Authority | Raimus U.K., Ltd.
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	500,000
Financial resources requirement	100,000
Excess financial resources	400,000
U.K. Financial Services Authority | Cowen International Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	5,000,000
Financial resources requirement	2,100,000
Excess financial resources	2,900,000
H.K. Securities and Futures Commission | Cowen and Company (Asia) Limited
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	800,000
Financial resources requirement	400,000
Excess financial resources	$ 400,000