Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Employee Stock Option [Member]	Jun. 30, 2011 Employee Stock Option [Member]	Jun. 30, 2012 Employee Stock Option [Member]	Jun. 30, 2011 Employee Stock Option [Member]	Jun. 30, 2012 Restricted Stock [Member]	Jun. 30, 2011 Restricted Stock [Member]	Jun. 30, 2012 Restricted Stock [Member]	Jun. 30, 2011 Restricted Stock [Member]	Jun. 30, 2012 Equity Plans [Member]	Jun. 30, 2012 Equity Plans [Member] Non-employee Director [Member] Restricted Stock Units (RSUs) [Member]	Jun. 30, 2012 Common Stock Class A	Dec. 31, 2011 Common Stock Class A	Jun. 30, 2012 Common Stock Class A Restricted Stock [Member]
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding															114,208,268	114,047,637
Share-based compensation, shares outstanding																	420,276
Shares available for issuance under compensation plan, in shares													1,400,000	336,895
Computation of earnings per share:
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ (7,946)	$ 20,037	$ (3,951)	$ 20,119
Shares for basic and diluted calculations:
Weighted average shares used in basic computation	114,561,000	76,330,000	114,420,000	75,600,000
Shares attributable to share-based payment awards					0	0	0	0	0	1,568,000	0	1,289,000
Weighted average shares used in diluted computation	114,561,000	77,898,000	114,420,000	76,889,000
Earnings (loss) per share:
Basic	$ (0.07)	$ 0.26	$ (0.03)	$ 0.27
Diluted	$ (0.07)	$ 0.26	$ (0.03)	$ 0.26