Share-Based Compensation and Employee Ownership Plans Deferred Compensation (Details) (Cowen Group, Inc. 2010 Equity and Incentive Plan, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Deferred Cash Award
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash awards granted	29.3
Deferred cash awards, unrecognized compensation expense	33.8
Maximum \| Deferred Cash Award
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash award, interest rate	0.75%
Minimum \| Deferred Cash Award
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash award, interest rate	0.70%
Deferred Cash Award
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash awards, vesting period	5 years

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

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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