Acquisitions Narrative (Details) (USD $)	3 Months Ended								12 Months Ended					0 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 ATM Group and Cowen Equity Finance LP [Member]	Dec. 31, 2011 ATM Group and Cowen Equity Finance LP [Member]	Jun. 28, 2011 LaBranche and Co Inc. [Member]	Jun. 30, 2011 LaBranche and Co Inc. [Member]	Dec. 31, 2011 LaBranche and Co Inc. [Member]	Dec. 31, 2012 Minimum ATM Group and Cowen Equity Finance LP [Member]	Dec. 31, 2012 Maximum ATM Group and Cowen Equity Finance LP [Member]
Business Acquisition [Line Items]
Business Acquisition, Cost of Acquired Entity, Cash Paid												$ 10,900,000
Conversion ratio, shares of Company's stock issued per share of Acquiree's stock														0.9980
Shares issued in acquisition, value														156,000,000
Closing stock price	$ 2.45				$ 2.59				$ 2.45	$ 2.59				$ 3.82
Shares outstanding, in shares														40,931,997
Net assets acquired												10,285,000		175,982,000
Non compete agreements and covenants with limiting conditions acquired recognized separately												167,000		2,310,000
Total purchase price												18,969,000		156,048,000
Bargain purchase gain recognized									0	22,244,000	0	8,517,000		22,244,000	22,200,000
Acquisition related costs													300,000			3,300,000
Revenues												6,200,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	(9,349,000)	(10,585,000)	(7,946,000)	3,995,000	(79,896,000)	(48,233,000)	20,037,000	82,000	(23,885,000)	(108,010,000)	(45,417,000)	1,600,000
Intangibles												9,900,000		2,770,000
Noncompete Agreements														2,500,000
Business Acquisition, Cost of Acquired Entity, Liabilities Incurred												8,100,000
Business Acquisition, Contingent Consideration, Potential Cash Payment																	5,000,000	13,400,000
Business Acquisition, Purchase Price Allocation, Goodwill, Expected Tax Deductible Amount	$ 2,200,000								$ 2,200,000