Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Computation of earnings per share:
Net income (loss)	$ (7,995)	$ 19,418
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	(4,297)	2,720
Net income (loss) attributable to Cowen Group, Inc.	(3,698)	16,698
Preferred Stock Dividends	1,698	0
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ (5,396)	$ 16,698
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	106,364,000	112,053,000
Weighted average shares used in diluted computation, shares	106,364,000	118,590,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ (0.05)	$ 0.15
Earnings Per Share, Diluted (in dollars per share)	$ (0.05)	$ 0.14
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	0
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	411,000
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	119,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	6,007,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	106,078,205	111,168,897	105,604,658	111,691,199.000
Common stock, restricted shares, shares	497,570		497,570