Earnings Per Share (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Warrants and Rights	Sep. 30, 2013 Warrants and Rights	Sep. 30, 2014 Warrants and Rights	Sep. 30, 2013 Warrants and Rights	Sep. 30, 2014 Stock Options	Sep. 30, 2013 Stock Options	Sep. 30, 2014 Stock Options	Sep. 30, 2013 Stock Options	Sep. 30, 2014 Stock Appreciation Rights (SARs)	Sep. 30, 2013 Stock Appreciation Rights (SARs)	Sep. 30, 2014 Stock Appreciation Rights (SARs)	Sep. 30, 2013 Stock Appreciation Rights (SARs)	Sep. 30, 2014 Restricted Stock	Sep. 30, 2013 Restricted Stock	Sep. 30, 2014 Restricted Stock	Sep. 30, 2013 Restricted Stock	Sep. 30, 2014 Common Stock Class A	Dec. 31, 2013 Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares																					113,634,803	115,026,633
Common stock, restricted shares, shares																					424,479	482,522
Computation of earnings per share:
Net income (loss)	$ 10,529,000	$ 8,370,000	$ 38,154,000	$ 12,637,000
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	4,006,000	4,759,000	13,409,000	10,509,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ 6,523,000	$ 3,611,000	$ 24,745,000	$ 2,128,000
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	114,969,000	118,359,000	115,407,000	116,012,000
Shares attributable to share-based payment awards, shares					0	0	0	0	0	0	0	0	59,000	311,000	57,000	296,000	3,773,000	4,038,000	4,563,000	3,583,000
Weighted average shares used in diluted computation, shares	118,801,000	122,708,000	120,027,000	119,891,000
Earnings (loss) per share:
Earnings Per Share, Basic	$ 0.06	$ 0.03	$ 0.21	$ 0.02
Earnings Per Share, Diluted	$ 0.05	$ 0.03	$ 0.21	$ 0.02