Share-Based Compensation and Employee Ownership Plans Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended														1 Months Ended	3 Months Ended
	Mar. 31, 2013 Minimum	Mar. 31, 2013 Maximum	Mar. 31, 2013 Equity Plans	Mar. 31, 2012 Equity Plans	Mar. 31, 2013 Equity Plans Employee Stock Option Minimum	Mar. 31, 2013 Equity Plans Employee Stock Option Maximum	Mar. 31, 2013 Equity Plans Restricted Shares and Restricted Stock Units (RSUs)	Mar. 31, 2012 Equity Plans Restricted Shares and Restricted Stock Units (RSUs)	Mar. 31, 2013 Equity Plans Stock Appreciation Rights (SARs)	Mar. 31, 2013 Equity Plans Restricted Stock Minimum	Mar. 31, 2013 Equity Plans Restricted Stock Maximum	Mar. 31, 2013 Equity Plans Restricted Stock Units (RSUs) Non-employee Director	Mar. 31, 2012 RCG Grants	Mar. 31, 2013 RCG Grants Restricted Stock	Feb. 29, 2012 Deferred Cash Award Cowen Group, Inc. 2010 Equity and Incentive Plan	Mar. 31, 2013 Deferred Cash Award Cowen Group, Inc. 2010 Equity and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized under compensation plan, in shares			17,725,000
Stock compensation award, vesting period					2 years	5 years				2 years	5 years			3 years
Stock options, initial term			7 years
SAR's, initial term									5 years
Shares available for issuance under compensation plan, in shares			2,400,000									336,895
Deferred cash awards granted															$ 27.7
Deferred cash awards, vesting period	1 year	3 years														5 years
Deferred cash award, interest rate																0.75%
Deferred cash awards, unrecognized compensation expense																35.7
Stock-compensation expense recognized in connection with compensation plan			4.8	4.2									1.3
Tax benefit of stock-compensation expense recognized in connection with compensation plan			$ 2.0	$ 2.2									$ 0.5
Units awarded							5,866,349	8,381,939	400,000,000			0
Vested, shares							(1,036,040)	(4,855,489)	0			0