Share-Based Compensation and Employee Ownership Plans - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended							3 Months Ended	9 Months Ended			0 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2012 Equity Plans [Member]	Sep. 30, 2011 Equity Plans [Member]	Sep. 30, 2012 Equity Plans [Member]	Sep. 30, 2011 Equity Plans [Member]	Sep. 30, 2012 Equity Plans [Member] Employee Stock Option [Member] Minimum [Member]	Sep. 30, 2012 Equity Plans [Member] Employee Stock Option [Member] Maximum [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Shares and Restricted Stock Units (RSUs) [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Stock [Member] Minimum [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Stock [Member] Maximum [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Non-employee Director [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Non-employee Director [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Minimum [Member]	Sep. 30, 2012 Equity Plans [Member] Restricted Stock Units (RSUs) [Member] Maximum [Member]	Jan. 02, 2012 Cowen Group, Inc. 2010 Equity and Incentive Plan [Member]	Sep. 30, 2012 Cowen Group, Inc. 2010 Equity and Incentive Plan [Member]	Sep. 30, 2012 RCG Grants [Member]	Sep. 30, 2011 RCG Grants [Member]	Sep. 30, 2012 RCG Grants [Member]	Sep. 30, 2011 RCG Grants [Member]	Sep. 30, 2012 RCG Grants [Member] Restricted Stock [Member]	Sep. 30, 2012 Grants to Non-employee Directors [Member] Restricted Stock Units (RSUs) [Member]	Sep. 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	1,100,000		1,100,000							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	$ 5.7	$ 5.1	$ 16.2	$ 15.6												$ 1.3	$ 1.0	$ 4.4	$ 3.9
Tax benefit of stock-compensation expense recognized in connection with compensation plan	$ 4.4	$ 2.8	$ 9.0	$ 7.8												$ 0.5	$ 0.4	$ 1.7	$ 1.5
Shares granted to non-employee board members											257,004										0	257,004
Shares vested in period							4,337,252			0	108,237									0