Share-Based Compensation and Employee Ownership Plans Deferred Cash (Details) - Cowen Group, Inc. 2010 Equity and Incentive Plan - Deferred Cash Award $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash awards granted	$ 25.9
Deferred cash awards, vesting period	4 years
Deferred cash awards, unrecognized compensation expense	$ 32.9
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%