Share-Based Compensation and Employee Ownership Plans Deferred Cash (Details) - Cowen Group, Inc. 2010 Equity and Incentive Plan - Deferred Cash Award $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred cash awards granted	$ 50.1
Deferred cash awards, vesting period	4 years
Deferred cash award, interest rate	0.70%
Deferred cash awards, unrecognized compensation expense	$ 100.7