Acquisitions and Divestitures (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 23, 2016
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	$ 100,000	$ 100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	15,100,000	10,000,000
Proceeds from Divestiture of Businesses, Net of Cash Divested	17,303,000	$ 0	$ 0
CRT business
Business Acquisition [Line Items]
Payment to acquire business	6,300,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	8,000,000
Business Acquisition, Transaction Costs	400,000
Contractual Obligation	6,000,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	20,500,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	4,100,000
Cowen Prime and Cowen Prime Trading
Business Acquisition [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	40,200,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	3,700,000
Revenues	496,543,000
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ 40,613,000
Net income per common share - Basic (in dollars per share)	$ 1.48
Net income per common share - Diluted (in dollars per share)	$ 1.40
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