Regulatory Requirements	6 Months Ended
Jun. 30, 2012
Brokers and Dealers [Abstract]
Regulatory Requirements	Regulatory Requirements As registered broker‑dealers, Cowen and Company, Cowen Capital LLC (formerly known as LaBranche Capital, LLC) and ATM USA are subject to the SEC's Uniform Net Capital Rule 15c3-1 (the “Rule”), which requires the maintenance of minimum net capital. Under the alternative method permitted by the Rule, Cowen and Company's minimum net capital requirement, as defined, is $1.0 million. Under the basic method permitted by the Rule, Cowen Capital LLC is required to maintain minimum net capital, as defined, equivalent to the greater of $1.0 million or 6.667% of aggregate indebtedness. ATM USA is required to maintain minimum net capital, as defined, equivalent to the greater of $5,000 or 6.667% of aggregate indebtedness. The broker-dealers are not permitted to withdraw equity if certain minimum net capital requirements are not met. As of June 30, 2012, Cowen and Company had total net capital of approximately $31.8 million, which was approximately $30.8 million in excess of its minimum net capital requirement of $1.0 million. As of June 30, 2012, Cowen Capital, LLC had total net capital of approximately $3.1 million, which was approximately $2.1 million in excess of its minimum net capital requirement of $1.0 million. As of June 30, 2012, ATM USA had total net capital of approximately $0.2 million, which was approximately $0.1 million in excess of its minimum net capital requirement of $0.1 million. Cowen and Company, Cowen Capital LLC and ATM USA are exempt from the provisions of Rule 15c3-3 under the Securities Exchange Act of 1934 as its activities are limited to those set forth in the conditions for exemption appearing in paragraph (k)(2)(ii) of the Rule. Proprietary accounts of introducing brokers (“PAIB”) held at the clearing broker are considered allowable assets for net capital purposes, pursuant to agreements between Cowen and Company, Cowen Capital LLC and ATM USA and the clearing broker, which require, among other things, that the clearing broker performs computations for PAIB and segregates certain balances on behalf of Cowen and Company, Cowen Capital LLC and ATM USA, if applicable. Ramius UK Ltd. (“Ramius UK”) and Cowen International Limited (“CIL”) are subject to the capital requirements of the Financial Services Authority (“FSA”) of the UK. Financial Resources, as defined, must exceed the requirement of the FSA. As of June 30, 2012, Ramius UK's Financial Resources of $0.6 million exceeded its minimum requirement of $0.2 million by $0.4 million. As of June 30, 2012, CIL's Financial Resources of $4.0 million exceeded its minimum requirement of $2.2 million by $1.8 million. During the first quarter of 2012, due to the discontinuation of the LaBranche business, the firm decided to close the operations of CITL (formerly known as LaBranche Structured Products Europe Limited), a registered broker-dealer. On March 8, 2012, CITL was de-registered from the FSA. As of March 31, 2012, CITL was no longer subject to the regulatory capital requirements of the FSA in the United Kingdom. Cowen and Company (Asia) Limited (“CCAL”) (formerly known as Cowen Latitude Advisors Limited) is subject to the financial resources requirements of the Securities and Futures Commission (“SFC”) of Hong Kong. Financial Resources, as defined, must exceed the Total Financial Resources requirement of the SFC. As of June 30, 2012, CCAL's Financial Resources of $0.6 million exceeded the minimum requirement of $0.4 million by $0.2 million. In connection with the Company's decision to discontinue the LaBranche business, the Company decided to liquidate CSPH (formerly known as LaBranche Structured Products Hong Kong Limited), a registered broker-dealer. On June 11, 2012, CSPH was de-registered with the Hong Kong Securities and Futures (Financial Resources) Rules ("FRR"). As of June 30, 2012, CSPH was no longer subject to the regulatory requirements of the FRR in Hong Kong.

Regulatory Requirements

As registered broker‑dealers, Cowen and Company, Cowen Capital LLC (formerly known as LaBranche Capital, LLC) and ATM USA are subject to the SEC's Uniform Net Capital Rule 15c3-1 (the “Rule”), which requires the maintenance of minimum net capital. Under the alternative method permitted by the Rule, Cowen and Company's minimum net capital requirement, as defined, is $1.0 million. Under the basic method permitted by the Rule, Cowen Capital LLC is required to maintain minimum net capital, as defined, equivalent to the greater of $1.0 million or 6.667% of aggregate indebtedness. ATM USA is required to maintain minimum net capital, as defined, equivalent to the greater of $5,000 or 6.667% of aggregate indebtedness. The broker-dealers are not permitted to withdraw equity if certain minimum net capital requirements are not met. As of June 30, 2012, Cowen and Company had total net capital of approximately $31.8 million, which was approximately $30.8 million in excess of its minimum net capital requirement of $1.0 million. As of June 30, 2012, Cowen Capital, LLC had total net capital of approximately $3.1 million, which was approximately $2.1 million in excess of its minimum net capital requirement of $1.0 million. As of June 30, 2012, ATM USA had total net capital of approximately $0.2 million, which was approximately $0.1 million in excess of its minimum net capital requirement of $0.1 million.

Cowen and Company, Cowen Capital LLC and ATM USA are exempt from the provisions of Rule 15c3-3 under the Securities Exchange Act of 1934 as its activities are limited to those set forth in the conditions for exemption appearing in paragraph (k)(2)(ii) of the Rule.

Proprietary accounts of introducing brokers (“PAIB”) held at the clearing broker are considered allowable assets for net capital purposes, pursuant to agreements between Cowen and Company, Cowen Capital LLC and ATM USA and the clearing broker, which require, among other things, that the clearing broker performs computations for PAIB and segregates certain balances on behalf of Cowen and Company, Cowen Capital LLC and ATM USA, if applicable.

Ramius UK Ltd. (“Ramius UK”) and Cowen International Limited (“CIL”) are subject to the capital requirements of the Financial Services Authority (“FSA”) of the UK. Financial Resources, as defined, must exceed the requirement of the FSA. As of June 30, 2012, Ramius UK's Financial Resources of $0.6 million exceeded its minimum requirement of $0.2 million by $0.4 million. As of June 30, 2012, CIL's Financial Resources of $4.0 million exceeded its minimum requirement of $2.2 million by $1.8 million.

During the first quarter of 2012, due to the discontinuation of the LaBranche business, the firm decided to close the operations of CITL (formerly known as LaBranche Structured Products Europe Limited), a registered broker-dealer. On March 8, 2012, CITL was de-registered from the FSA. As of March 31, 2012, CITL was no longer subject to the regulatory capital requirements of the FSA in the United Kingdom.

Cowen and Company (Asia) Limited (“CCAL”) (formerly known as Cowen Latitude Advisors Limited) is subject to the financial resources requirements of the Securities and Futures Commission (“SFC”) of Hong Kong. Financial Resources, as defined, must exceed the Total Financial Resources requirement of the SFC. As of June 30, 2012, CCAL's Financial Resources of $0.6 million exceeded the minimum requirement of $0.4 million by $0.2 million.

In connection with the Company's decision to discontinue the LaBranche business, the Company decided to liquidate CSPH (formerly known as LaBranche Structured Products Hong Kong Limited), a registered broker-dealer. On June 11, 2012, CSPH was de-registered with the Hong Kong Securities and Futures (Financial Resources) Rules ("FRR"). As of June 30, 2012, CSPH was no longer subject to the regulatory requirements of the FRR in Hong Kong.