Share-Based Compensation and Employee Ownership Plans - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended							3 Months Ended	6 Months Ended			0 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2012 Equity Plans [Member]	Jun. 30, 2011 Equity Plans [Member]	Jun. 30, 2012 Equity Plans [Member]	Jun. 30, 2011 Equity Plans [Member]	Jun. 30, 2012 Equity Plans [Member] Employee Stock Option [Member] Minimum [Member]	Jun. 30, 2012 Equity Plans [Member] Employee Stock Option [Member] Maximum [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Shares and Restricted Stock Units (RSUs) [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Stock [Member] Minimum [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Stock [Member] Maximum [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Non-employee Director [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Non-employee Director [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Minimum [Member]	Jun. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Maximum [Member]	Jan. 02, 2012 Cowen Group, Inc. 2010 Equity and Incentive Plan [Member]	Jun. 30, 2012 Cowen Group, Inc. 2010 Equity and Incentive Plan [Member]	Jun. 30, 2012 RCG Grants [Member]	Jun. 30, 2011 RCG Grants [Member]	Jun. 30, 2012 RCG Grants [Member]	Jun. 30, 2011 RCG Grants [Member]	Jun. 30, 2012 RCG Grants [Member] Restricted Stock [Member]	Jun. 30, 2012 Grants to Non-employee Directors [Member] Restricted Stock Units (RSUs) [Member]	Jun. 30, 2012 Grants to Non-employee Directors [Member] Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized under compensation plan, in shares	17,725,000		17,725,000
Stock compensation award, vesting period					2 years	5 years		2 years	5 years			2 years	5 years							3 years
Stock options, initial term			7 years
Shares available for issuance under compensation plan, in shares	1,400,000		1,400,000							336,895	336,895
Additional shares authorized under compensation plan, in shares														8,200,000
Shares available under compensation plan, as a percentage of outstanding stock															7.50%
Stock-compensation expense recognized in connection with compensation plan	$ 5.9	$ 4.9	$ 10.4	$ 10.6												$ 1.6	$ 1.5	$ 3.1	$ 2.9
Tax benefit of stock-compensation expense recognized in connection with compensation plan	$ 2.3	$ 2.4	$ 4.6	$ 5.1												$ 0.6	$ 0.6	$ 1.2	$ 1.1
Shares granted to non-employee board members										250,002	257,004										7,002	7,002
Shares vested in period							2,789,649			108,237	108,237									0