Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Computation of earnings per share:
Net income (loss)	$ (6,002)	$ 10,529	$ 24,048	$ 38,154
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	4,344	4,006	10,980	13,409
Net income (loss) attributable to Cowen Group, Inc.	(10,346)	6,523	13,068	24,745
Preferred Stock Dividends	1,603	0	2,358	0
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ (11,949)	$ 6,523	$ 10,710	$ 24,745
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	109,191,000	114,969,000	111,050,000	115,407,000
Weighted average shares used in diluted computation, shares	109,191,000	118,801,000	117,249,000	120,027,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ (0.11)	$ 0.06	$ 0.10	$ 0.21
Earnings Per Share, Diluted (in dollars per share)	$ (0.11)	$ 0.05	$ 0.09	$ 0.21
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0	19,000	0
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0	266,000	0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	59,000	152,000	57,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	3,773,000	5,762,000	4,563,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	107,098,068	113,634,803	107,098,068	113,634,803	111,691,199	115,026,633
Common stock, restricted shares, shares	497,570		497,570		424,479