Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Computation of earnings per share:
Net income (loss)									$ 58,975	$ 182,780	$ 17,840
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	$ 4,266	$ 4,344	$ 3,916	$ 2,720	$ 2,155	$ 4,006	$ 5,216	$ 4,187	15,246	15,564	13,193
Net income (loss) attributable to Cowen Group, Inc.	30,661	(10,346)	6,716	16,698	$ 142,471	$ 6,523	$ 8,382	$ 9,840	43,729	167,216	4,647
Preferred Stock Dividends	1,717	1,603	755	0					4,075	0	0
Net income (loss) attributable to Cowen Group, Inc. common stockholders	$ 28,944	$ (11,949)	$ 5,961	$ 16,698					$ 39,654	$ 167,216	$ 4,647
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	107,236,000	109,191,000	111,915,000	112,053,000	113,492,000	114,969,000	115,569,000	115,680,000	110,090,000	114,926,000	116,703,000
Weighted average shares used in diluted computation, shares	112,730,000	109,191,000	118,226,000	118,590,000	118,222,000	118,801,000	120,199,000	122,898,000	116,174,000	119,486,000	121,117,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share)	$ 0.27	$ (0.11)	$ 0.05	$ 0.15	$ 1.26	$ 0.06	$ 0.07	$ 0.09	$ 0.36	$ 1.45	$ 0.04
Earnings Per Share, Diluted (in dollars per share)	$ 0.26	$ (0.11)	$ 0.05	$ 0.14	$ 1.21	$ 0.05	$ 0.07	$ 0.08	$ 0.34	$ 1.40	$ 0.04
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									13,000	0	0
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									263,000	0	0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									140,000	60,000	306,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									5,668,000	4,500,000	4,108,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	105,604,658				111,691,199				105,604,658	111,691,199	115,026,633	112,447,892
Common stock, restricted shares, shares	497,570				424,479				497,570	424,479