Acquisition - Narrative (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 LaBranche and Co Inc. [Member]	Jun. 30, 2011 LaBranche and Co Inc. [Member]	Jun. 30, 2012 LaBranche and Co Inc. [Member]	Jun. 30, 2011 LaBranche and Co Inc. [Member]	Jun. 28, 2011 LaBranche and Co Inc. [Member]
Business Acquisition [Line Items]
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	$ 2.66		$ 2.66		$ 2.59					$ 3.82
Shares outstanding, in shares										40,931,997
Net assets acquired										175,982,000
Non compete agreements and covenants with limiting conditions acquired										2,310,000
Total purchase price										156,048,000
Bargain purchase gain recognized	0	22,244,000	0	22,244,000			22,244,000
Acquisition related costs							600,000		2,300,000
Revenues	59,470,000	58,679,000	116,949,000	122,924,000		200,000		200,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ (7,946,000)	$ 20,037,000	$ (3,951,000)	$ 20,119,000		$ 200,000		$ 200,000