Share-Based Compensation and Employee Ownership Plans - Deferred Compensation (Details) (Cowen Group, Inc. 2010 Equity and Incentive Plan [Member], Deferred Cash Award [Member], USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended
	Feb. 29, 2012	Sep. 30, 2012
Cowen Group, Inc. 2010 Equity and Incentive Plan [Member] | Deferred Cash Award [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash awards granted	$ 16.5
Deferred cash awards, vesting period		5 years
Deferred cash award, interest rate		0.75%
Deferred cash awards, unrecognized compensation expense		$ 13.2