Acquisitions Narrative (Details) (USD $)	3 Months Ended								12 Months Ended					0 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 ATM Group and Cowen Equity Finance LP [Member]	Dec. 31, 2012 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, 2013 Minimum ATM Group and Cowen Equity Finance LP [Member]	Dec. 31, 2013 Maximum ATM Group and Cowen Equity Finance LP [Member]
Business Acquisition [Line Items]
Business Acquisition, Cost of Acquired Entity, Cash Paid									$ 779,000	$ 0	$ 0		$ 10,900,000
Business Acquisition, Cost of Acquired Entity, Contingent Consideration													8,100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low																	3,500,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High																		13,800,000
Business Acquisition, Purchase Price Allocation, Goodwill, Expected Tax Deductible Amount												2,200,000
Business Combination, Acquisition Related Costs													300,000			3,300,000
Revenues	97,438,000	81,360,000	81,207,000	67,241,000	65,932,000	57,598,000	59,470,000	57,480,000	327,246,000	240,480,000	235,278,000	6,200,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	2,519,000	3,611,000	1,093,000	(2,576,000)	(9,349,000)	(10,585,000)	(7,946,000)	3,995,000	4,647,000	(23,885,000)	(108,010,000)	1,600,000
Business Acquisition, Purchase Price Allocation, Assets Acquired (Liabilities Assumed), Net													10,285,000	176,000,000
Non compete agreements and covenants with limiting conditions acquired recognized separately													167,000	2,300,000
Business Acquisition, Cost of Acquired Entity, Purchase Price														156,000,000
Bargain purchase gain									$ 0	$ 0	$ 22,244,000		$ 8,517,000		$ 22,200,000