Acquisition (Details) - USD ($)	4 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	$ 100,000	$ 100,000	$ 900,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	10,000,000	10,000,000	7,100,000
Goodwill Impairment		0	2,334,000	$ 0
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability		(200,000)	(2,055,000)	$ 0
Cowen Prime and Cowen Prime Trading
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability	3,600,000	3,600,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	0	0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	7,200,000	7,200,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	2,966,000	2,966,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Receivable from brokers	5,327,000	5,327,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Fixed Assets	644,000	644,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Fees Receivable	983,000	983,000
Intangible Assets, Net (Including Goodwill)	19,900,000	19,900,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	684,000	684,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Payable to brokers	(153,000)	(153,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Compensation Payable	(1,667,000)	(1,667,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(3,250,000)	(3,250,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	25,434,000	25,434,000
Business Acquisition, Transaction Costs	1,000,000	1,000,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	13,400,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	$ 1,400,000
Revenues		496,543,000	473,192,000
Net income (loss) attributable to Cowen Group, Inc. common stockholders		$ 40,613,000	$ 167,238,000
Net income per common share - Basic (in dollars per share)		$ 0.37	$ 1.45
Net income per common share - Diluted (in dollars per share)		$ 0.35	$ 1.39
Cowen Equity Finance
Business Acquisition [Line Items]
Impairment of Intangible Assets, Finite-lived			$ 900,000
Cash [Member] | Cowen Prime and Cowen Prime Trading
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		$ 42,300,000
Common Stock | Cowen Prime and Cowen Prime Trading
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		3,000,000
Broker-Dealer
Business Acquisition [Line Items]
Goodwill Impairment		$ 0	2,334,000
Broker-Dealer | Cowen Equity Finance
Business Acquisition [Line Items]
Goodwill Impairment			2,300,000
Orchard Square Partners
Business Acquisition [Line Items]
Assets under Management, Carrying Amount			420,800,000
Gain (Loss) on Disposition of Business			$ 4,500,000