Share-Based Compensation and Employee Ownership Plans Deferred Cash (Details) - Cowen Group, Inc. 2010 Equity and Incentive Plan - Deferred Cash Award $ in Millions	9 Months Ended
Sep. 30, 2015 USD ($)
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash awards granted	$ 12.5
Deferred cash awards, vesting period	4 years
Deferred cash awards, unrecognized compensation expense	$ 32.4
Minimum
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash award, interest rate	0.70%
Maximum
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash award, interest rate	0.75%

Deferred Compensation Arrangement with Individual, Interest Rate

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- Definition

Deferred Compensation Arrangement with Individual, Total Compensation Cost Not yet Recognized

+ References

No definition available.

+ Details

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- Definition

Amount of commitment made to pay deferred cash remuneration.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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