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Share-Based Compensation and Employee Ownership Plans Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	100,000
Equity Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-compensation expense recognized in connection with compensation plan	$ 8.3	$ 7.8
Tax benefit of stock-compensation expense recognized in connection with compensation plan	$ 1.8	$ 2.1
Equity Plans \| Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, initial term	7 years
Equity Plans \| Employee Stock Option \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation award, vesting period	2 years
Equity Plans \| Employee Stock Option \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation award, vesting period	5 years
Equity Plans \| Restricted Shares and Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	$ 86.1
Vested, shares	640,084	768,468
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	6,747,424	6,598,097	5,364,486	5,962,295
Equity Plans \| Restricted Stock \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation award, vesting period	2 years
Equity Plans \| Restricted Stock \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation award, vesting period	5 years
Equity Plans \| Restricted Stock Units (RSUs) \| Non-employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	216,912		216,912
Deferred Cash Award \| Cowen Group, Inc. 2010 Equity and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation award, vesting period	4 years