Share-Based Compensation and Employee Ownership Plans Deferred Compensation (Details) (Cowen Group, Inc. 2010 Equity and Incentive Plan, USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 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	$ 38.7
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