Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Computation of earnings per share:
Net income (loss)	$ 15,259	$ (6,002)	$ (19,897)	$ 24,048
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	18,478	4,344	(2,524)	10,980
Net income (loss) attributable to Cowen Group, Inc.	(3,219)	(10,346)	(17,373)	13,068
Preferred stock dividends	1,698	1,603	5,094	2,358
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ (4,917)	$ (11,949)	$ (22,467)	$ 10,710
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	107,974,000	109,191,000	107,272,000	111,050,000
Weighted average shares used in diluted computation, shares	107,974,000	109,191,000	107,272,000	117,249,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ (0.05)	$ (0.11)	$ (0.21)	$ 0.10
Earnings Per Share, Diluted (in dollars per share)	$ (0.05)	$ (0.11)	$ (0.21)	$ 0.09
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0	0	19,000
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0	0	266,000
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0	0	152,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0	0	5,762,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	107,314,157	107,098,068	107,314,157	107,098,068	105,604,658	111,691,199.000
Common stock, restricted shares, shares	648,704		648,704		497,570