Acquisition (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 23, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Proceeds from Divestiture of Businesses, Net of Cash Divested		$ 17,303,000	$ 0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	$ 100,000	100,000			$ 100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	16,300,000	16,300,000			$ 10,000,000
CRT business
Business Acquisition [Line Items]
Payment to acquire business		6,300,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	0	0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	8,000,000	8,000,000
Business Acquisition, Transaction Costs	400,000	400,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	9,700,000	13,100,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	1,400,000	1,900,000
Cowen Prime and Cowen Prime Trading
Business Acquisition [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	8,600,000	30,700,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	$ 400,000	2,800,000
Revenues		385,932,000
Net income (loss) attributable to Cowen Group, Inc. common stockholders		$ 10,672,000
Net income per common share - Basic (in dollars per share)		$ 0.10
Net income per common share - Diluted (in dollars per share)		$ 0.09
Alternative Solutions [Member]
Business Acquisition [Line Items]
Assets under Management, Carrying Amount				$ 2,500,000,000
Proceeds from Divestiture of Businesses, Net of Cash Divested		$ 17,300,000