Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Computation of earnings per share:
Net income (loss) from continuing operations	$ 5,203	$ 8,370	$ 3,348	$ 919	$ (8,195)	$ (11,618)	$ (10,380)	$ 6,236	$ 17,840	$ (23,957)	$ (78,537)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	2,684	4,759	2,255	3,495	1,154	(1,033)	(2,434)	2,241	13,193	(72)	5,827
Net income (loss) attributable to Cowen Group, Inc. stockholders									4,647	(23,885)	(84,364)
Net income (loss) from discontinued operations, net of tax									$ 0	$ 0	$ (23,646)
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	117,514,000	118,359,000	117,235,000	113,798,000	113,939,000	114,989,000	114,561,000	114,281,000	116,703,000	114,400,000	95,532,000
Weighted average shares used in diluted computation, shares	123,573,000	122,708,000	120,901,000	113,798,000	113,939,000	114,989,000	114,561,000	115,663,000	121,117,000	114,400,000	95,532,000
Earnings (loss) per share:
Income (loss) from continuing operations, basic (in dollars per share)	$ 0.02	$ 0.03	$ 0.01	$ (0.02)	$ (0.08)	$ (0.09)	$ (0.07)	$ 0.03	$ 0.04	$ (0.21)	$ (0.88)
Income (loss) from discontinued operations, basic (in dollars per share)									$ 0.00	$ 0.00	$ (0.25)
Income (loss) from continuing operations, diluted (in dollars per share)	$ 0.02	$ 0.03	$ 0.01	$ (0.02)	$ (0.08)	$ (0.09)	$ (0.07)	$ 0.03	$ 0.04	$ (0.21)	$ (0.88)
Income (loss) from discontinued operations, diluted (in dollars per share)									$ 0.00	$ 0.00	$ (0.25)
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									0	0	0
Stock Appreciation Rights (SARs)
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									306,000	0	0
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares									4,108,000	0	0
Non-employee Director | Equity Plans | Restricted Stock Units (RSUs)
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, restricted shares, shares	482,522								482,522
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares	115,026,633				112,447,892				115,026,633	112,447,892
Common stock, restricted shares, shares	482,522				336,895				482,522	336,895