Regulatory Requirements (Details) (USD $)	Sep. 30, 2012
U.S. Securities and Exchange Commission [Member] | Cowen and Company [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	$ 1,000,000
Net capital	31,100,000
Excess net capital under alternative method	30,100,000
U.S. Securities and Exchange Commission [Member] | Cowen and Company [Member] | Minimum [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital requirement under alternative method	1,000,000
U.S. Securities and Exchange Commission [Member] | Cowen Capital, LLC [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital	3,100,000
Excess capital	2,100,000
Minimum net capital required	1,000,000
U.S. Securities and Exchange Commission [Member] | Cowen Capital, LLC [Member] | Minimum [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	1,000,000
Net capital requirement as a percentage of indebtedness	6.667%
U.S. Securities and Exchange Commission [Member] | ATM USA [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Net capital	600,000
Excess capital	500,000
Minimum net capital required	100,000
U.S. Securities and Exchange Commission [Member] | ATM USA [Member] | Minimum [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Minimum net capital required	5,000
Net capital requirement as a percentage of indebtedness	6.667%
U.K. Financial Services Authority [Member] | Raimus U.K., Ltd. [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	600,000
Financial resources requirement	200,000
Excess financial resources	400,000
U.K. Financial Services Authority [Member] | Cowen International Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	4,500,000
Financial resources requirement	2,300,000
Excess financial resources	2,200,000
H.K. Securities and Futures Commission [Member] | Cowen and Company (Asia) Limited [Member]
Regulatory Requirements for Broker-Dealers [Line Items]
Financial resources	700,000
Financial resources requirement	400,000
Excess financial resources	$ 300,000