Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Computation of earnings per share:
Net income (loss)	$ (27,161)	$ 10,632	$ (35,156)	$ 30,050
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	(16,705)	3,916	(21,002)	6,636
Net income (loss) attributable to Cowen Group, Inc.	(10,456)	6,716	(14,154)	23,414
Preferred stock dividends	1,698	755	3,396	755
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ (12,154)	$ 5,961	$ (17,550)	$ 22,659
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	107,471,000	111,915,000	106,918,000	111,987,000
Weighted average shares used in diluted computation, shares	107,471,000	118,226,000	106,918,000	118,316,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ (0.11)	$ 0.05	$ (0.16)	$ 0.20
Earnings Per Share, Diluted (in dollars per share)	$ (0.11)	$ 0.05	$ (0.16)	$ 0.19
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	25,000	0	27,000
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	276,000	0	261,000
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	170,000	0	148,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	0	5,840,000	0	5,893,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	107,206,626	109,993,262	107,206,626	109,993,262	105,604,658	111,691,199.000
Common stock, restricted shares, shares	648,704		648,704		497,570