Segment Reporting (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012 segment		Sep. 30, 2011		Sep. 30, 2012 segment		Sep. 30, 2011
Segment Reporting Information [Line Items]
Number of business segments, in segments	2				2
Revenues
Investment banking	$ 18,666,000		$ 10,816,000		$ 50,550,000		$ 39,841,000
Brokerage	22,701,000		25,670,000		71,282,000		77,868,000
Management fees	8,866,000		14,778,000		28,515,000		37,799,000
Incentive income	1,416,000		(1,823,000)		2,687,000		3,233,000
Interest and dividends	3,605,000		10,930,000		14,845,000		21,330,000
Reimbursement from affiliates	1,370,000		978,000		3,796,000		2,968,000
Other revenue	701,000		309,000		2,398,000		1,230,000
Expenses
Employee compensation and benefits	47,322,000		43,651,000		137,102,000		132,313,000
Interest and dividends	1,957,000		1,904,000		6,888,000		7,627,000
Total expenses	79,054,000		81,030,000		233,218,000		244,486,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	12,510,000		(21,989,000)		41,969,000		(4,631,000)
Total other income (loss)	10,001,000		(24,787,000)		43,406,000		21,330,000
Income taxes expense / (benefit)	163,000		72,000		496,000		(17,720,000)
Net income (loss)	(11,618,000)		(49,017,000)		(15,761,000)		(25,640,000)
Net income (loss) from discontinued operations, net of tax	0		(5,087,000)		0		(5,087,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(10,585,000)		(48,234,000)		(14,536,000)		(28,113,000)
Reported Under U.S. GAAP [Member]
Revenues
Investment banking	18,666,000		10,816,000		50,550,000		39,841,000
Brokerage	22,701,000		25,670,000		71,282,000		77,868,000
Management fees	8,866,000		14,778,000		28,515,000		37,799,000
Incentive income	1,416,000		(1,823,000)		2,687,000		3,233,000
Investment Income	0		0		0		0
Interest and dividends	3,605,000		10,930,000		14,845,000		21,330,000
Reimbursement from affiliates	1,370,000		978,000		3,796,000		2,968,000
Other revenue	701,000		309,000		2,398,000		1,230,000
Total revenues	57,598,000		61,959,000		174,547,000		184,883,000
Expenses
Employee compensation and benefits	47,322,000		43,651,000		137,102,000		132,313,000
Interest and dividends	1,957,000		1,904,000		6,888,000		7,627,000
Non-compensation expenses - Fixed	0		0		0		0
Non-compensation expenses - Variable	0		0		0		0
Non-compensation expenses	29,445,000		34,340,000		87,889,000		101,911,000
Reimbursement from affiliates	0		0		0		0
Total expenses	79,054,000		81,030,000		233,218,000		244,486,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	12,510,000		(21,989,000)		41,969,000		(4,631,000)
Bargain purchase gain							22,244,000	[1]
Total other income (loss)	10,001,000		(24,787,000)		43,406,000		21,330,000
Income (loss) before income taxes and non-controlling interests	(11,455,000)		(43,858,000)		(15,265,000)		(38,273,000)
Income taxes expense / (benefit)	163,000		72,000		496,000		(17,720,000)
Net income (loss)	(11,618,000)		(43,930,000)		(15,761,000)		(20,553,000)
Net income (loss) from discontinued operations, net of tax			(5,087,000)				(5,087,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	1,033,000		783,000		1,225,000		(2,473,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(10,585,000)		(48,234,000)		(14,536,000)		(28,113,000)
Operating Segments [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking	18,666,000		10,816,000		50,550,000		39,841,000
Brokerage	22,701,000		25,670,000		71,282,000		77,868,000
Management fees	13,426,000		18,476,000		42,032,000		48,062,000
Incentive income	1,699,000		(643,000)		8,303,000		10,217,000
Investment Income	9,152,000		(17,028,000)		38,543,000		22,881,000
Interest and dividends	0		0		0		0
Reimbursement from affiliates	0		0		0		0
Other revenue	381,000		(87,000)		694,000		486,000
Total revenues	66,025,000		37,204,000		211,404,000		199,355,000
Expenses
Employee compensation and benefits	46,249,000		42,198,000		133,803,000		127,302,000
Interest and dividends	131,000		182,000		278,000		617,000
Non-compensation expenses - Fixed	24,016,000		25,812,000		69,748,000		73,675,000
Non-compensation expenses - Variable	5,720,000		9,925,000		19,733,000		31,379,000
Non-compensation expenses	0		0		0		0
Reimbursement from affiliates	(1,467,000)		(1,028,000)		(4,018,000)		(3,186,000)
Total expenses	74,649,000		77,089,000		219,544,000		229,787,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Bargain purchase gain							0	[1]
Total other income (loss)	0		0		0		0
Income (loss) before income taxes and non-controlling interests	(8,624,000)		(39,885,000)		(8,140,000)		(30,432,000)
Income taxes expense / (benefit)	0		0		0		0
Net income (loss)	(8,624,000)		(39,885,000)		(8,140,000)		(30,432,000)
Net income (loss) from discontinued operations, net of tax			0				0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(301,000)		(810,000)		(900,000)		(2,750,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(8,925,000)		(40,695,000)		(9,040,000)		(33,182,000)
Alternative Investment [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking	0		0		0		0
Brokerage	0		306,000		0		380,000
Management fees	13,426,000		18,476,000		42,032,000		48,062,000
Incentive income	1,699,000		(643,000)		8,303,000		10,217,000
Investment Income	6,864,000		(13,795,000)		30,361,000		20,027,000
Interest and dividends	0		0		0		0
Reimbursement from affiliates	0		0		0		0
Other revenue	168,000		41,000		508,000		1,015,000
Total revenues	22,157,000		4,385,000		81,204,000		79,701,000
Expenses
Employee compensation and benefits	11,759,000		10,724,000		42,705,000		41,595,000
Interest and dividends	93,000		52,000		123,000		157,000
Non-compensation expenses - Fixed	8,564,000		7,445,000		24,093,000		23,163,000
Non-compensation expenses - Variable	1,433,000		4,315,000		3,881,000		13,002,000
Non-compensation expenses	0		0		0		0
Reimbursement from affiliates	(1,467,000)		(1,028,000)		(4,018,000)		(3,186,000)
Total expenses	20,382,000		21,508,000		66,784,000		74,731,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Bargain purchase gain							0	[1]
Total other income (loss)	0		0		0		0
Income (loss) before income taxes and non-controlling interests	1,775,000		(17,123,000)		14,420,000		4,970,000
Income taxes expense / (benefit)	0		0		0		0
Net income (loss)	1,775,000		(17,123,000)		14,420,000		4,970,000
Net income (loss) from discontinued operations, net of tax			0				0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(301,000)		(810,000)		(900,000)		(2,750,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	1,474,000		(17,933,000)		13,520,000		2,220,000
Broker-Dealer [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking	18,666,000	[2]	10,816,000	[3]	50,550,000	[4]	39,841,000	[5]
Brokerage	22,701,000	[2]	25,364,000	[3]	71,282,000	[4]	77,488,000	[5]
Management fees	0	[2]	0	[3]	0	[4]	0	[5]
Incentive income	0	[2]	0	[3]	0	[4]	0	[5]
Investment Income	2,288,000	[2]	(3,233,000)	[3]	8,182,000	[4]	2,854,000	[5]
Interest and dividends	0	[2]	0	[3]	0	[4]	0	[5]
Reimbursement from affiliates	0	[2]	0	[3]	0	[4]	0	[5]
Other revenue	213,000	[2]	(128,000)	[3]	186,000	[4]	(529,000)	[5]
Total revenues	43,868,000	[2]	32,819,000	[3]	130,200,000	[4]	119,654,000	[5]
Expenses
Employee compensation and benefits	34,490,000	[2]	31,474,000	[3]	91,098,000	[4]	85,707,000	[5]
Interest and dividends	38,000	[2]	130,000	[3]	155,000	[4]	460,000	[5]
Non-compensation expenses - Fixed	15,452,000	[2]	18,367,000	[3]	45,655,000	[4]	50,512,000	[5]
Non-compensation expenses - Variable	4,287,000	[2]	5,610,000	[3]	15,852,000	[4]	18,377,000	[5]
Non-compensation expenses	0	[2]	0	[3]	0	[4]	0	[5]
Reimbursement from affiliates	0	[2]	0	[3]	0	[4]	0	[5]
Total expenses	54,267,000	[2]	55,581,000	[3]	152,760,000	[4]	155,056,000	[5]
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0	[2]	0	[3]	0	[2]	0	[5]
Bargain purchase gain							0	[1],[5]
Total other income (loss)	0	[2]	0	[3]	0	[4]	0	[5]
Income (loss) before income taxes and non-controlling interests	(10,399,000)	[2]	(22,762,000)	[3]	(22,560,000)	[4]	(35,402,000)	[5]
Income taxes expense / (benefit)	0	[2]	0	[3]	0	[4]	0	[5]
Net income (loss)	(10,399,000)	[2]	(22,762,000)	[3]	(22,560,000)	[4]	(35,402,000)	[5]
Net income (loss) from discontinued operations, net of tax			0	[3]			0	[3]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	0	[2]	0	[3]	0	[4]	0	[5]
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(10,399,000)	[2]	(22,762,000)	[3]	(22,560,000)	[4]	(35,402,000)	[5]
Investment Income from Entity's Capital	2,300,000		5,100,000		8,200,000		1,300,000
Compensation Expense Related to Investment of Entity's Capital	800,000		1,700,000		2,700,000		200,000
Funds Consolidation [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Investment banking	0		0		0		0
Brokerage	0		0		0		0
Management fees	(362,000)		(428,000)		(1,150,000)		(1,407,000)
Incentive income	0		0		0		0
Investment Income	0		0		0		0
Interest and dividends	0		0		0		0
Reimbursement from affiliates	(97,000)		(49,000)		(222,000)		(217,000)
Other revenue	0		0		0		0
Total revenues	(186,000)		(176,000)		(898,000)		(1,010,000)
Expenses
Employee compensation and benefits	0		0		0		0
Interest and dividends	0		0		0		0
Non-compensation expenses - Fixed	0		0		0		0
Non-compensation expenses - Variable	0		0		0		0
Non-compensation expenses	0		0		0		0
Reimbursement from affiliates	0		0		0		0
Total expenses	330,000		1,135,000		1,339,000		2,635,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Bargain purchase gain							0	[1]
Total other income (loss)	(815,000)		(619,000)		119,000		3,031,000
Income (loss) before income taxes and non-controlling interests	(1,331,000)		(1,930,000)		(2,118,000)		(614,000)
Income taxes expense / (benefit)	0		0		0		0
Net income (loss)	(1,331,000)		(1,930,000)		(2,118,000)		(614,000)
Net income (loss) from discontinued operations, net of tax			0				0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	1,331,000		1,930,000		2,118,000		614,000
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	0		0		0		0
Other Adjustments [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Investment banking	0		0		0		0
Brokerage	0		0		0		0
Management fees	(4,198,000)	[6]	(3,270,000)	[6]	(12,367,000)	[6]	(8,856,000)	[6]
Incentive income	(283,000)	[6]	(1,180,000)	[6]	(5,616,000)	[6]	(6,984,000)	[6]
Investment Income	(9,152,000)	[7]	17,028,000	[7]	(38,543,000)	[7]	(22,881,000)	[7]
Interest and dividends	3,605,000	[7]	10,930,000	[7]	14,845,000	[7]	21,330,000	[7]
Reimbursement from affiliates	1,467,000	[8]	1,027,000	[8]	4,018,000	[8]	3,185,000	[8]
Other revenue	320,000	[7]	396,000	[7]	1,704,000	[7]	744,000	[7]
Total revenues	(8,241,000)		24,931,000		(35,959,000)		(13,462,000)
Expenses
Employee compensation and benefits	1,073,000		1,453,000		3,299,000		5,011,000
Interest and dividends	1,826,000	[7]	1,722,000	[7]	6,610,000	[7]	7,010,000	[7]
Non-compensation expenses - Fixed	(24,016,000)	[7],[9]	(25,812,000)	[7],[9]	(69,748,000)	[7],[9]	(73,675,000)	[7],[9]
Non-compensation expenses - Variable	(5,720,000)	[7],[9]	(9,925,000)	[7],[9]	(19,733,000)	[7],[9]	(31,379,000)	[7],[9]
Non-compensation expenses	29,445,000	[7],[9]	34,340,000	[7],[9]	87,889,000	[7],[9]	101,911,000	[7],[9]
Reimbursement from affiliates	1,467,000	[8]	1,028,000	[8]	4,018,000	[8]	3,186,000	[8]
Total expenses	4,075,000		2,806,000		12,335,000		12,064,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	12,510,000	[7]	(21,989,000)	[7]	41,969,000	[7]	(4,631,000)	[7]
Bargain purchase gain							22,244,000	[1]
Total other income (loss)	10,816,000		(24,168,000)		43,287,000		18,299,000
Income (loss) before income taxes and non-controlling interests	(1,500,000)		(2,043,000)		(5,007,000)		(7,227,000)
Income taxes expense / (benefit)	163,000	[8]	72,000	[8]	496,000	[8]	(17,720,000)	[8]
Net income (loss)	(1,663,000)		(2,115,000)		(5,503,000)		10,493,000
Net income (loss) from discontinued operations, net of tax			(5,087,000)	[10]			(5,087,000)	[10]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	3,000		(337,000)		7,000		(337,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(1,660,000)		(7,539,000)		(5,496,000)		5,069,000
Consolidated Funds [Member] | Reported Under U.S. GAAP [Member]
Revenues
Total revenues	273,000		301,000		474,000		614,000
Expenses
Total expenses	330,000		1,135,000		1,339,000		2,635,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	(2,509,000)		(2,798,000)		1,437,000		3,717,000
Consolidated Funds [Member] | Operating Segments [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Total revenues	0		0		0		0
Expenses
Total expenses	0		0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Consolidated Funds [Member] | Alternative Investment [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Total revenues	0		0		0		0
Expenses
Total expenses	0		0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Consolidated Funds [Member] | Broker-Dealer [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Total revenues	0	[2]	0	[3]	0	[4]	0	[5]
Expenses
Total expenses	0	[2]	0	[3]	0	[4]	0	[5]
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0	[2]	0	[3]	0	[2]	0	[5]
Consolidated Funds [Member] | Funds Consolidation [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Total revenues	273,000		301,000		474,000		614,000
Expenses
Total expenses	330,000		1,135,000		1,339,000		2,635,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	(815,000)		(619,000)		119,000		3,031,000
Consolidated Funds [Member] | Other Adjustments [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Total revenues	0		0		0		0
Expenses
Total expenses	0		0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	$ (1,694,000)		$ (2,179,000)		$ 1,318,000		$ 686,000

[1] Economic Income excludes the bargain purchase gain which resulted from the LaBranche acquisition.

[2] For the three months ended September 30, 2012 the Company has reflected $2.3 million of investment income and related compensation expense of $0.8 million within the broker-dealer segment in proportion to its capital.

[3] For the three months ended September 30, 2011, the Company has reflected an adjustment of $5.1 million of investment income and related compensation expense of $1.7 million out of the broker-dealer segment in proportion to its capital.

[4] For the nine months ended September 30, 2012, the Company has reflected $8.2 million of investment income and related compensation expense of $2.7 million within the broker-dealer segment in proportion to its capital.

[5] For the nine months ended September 30, 2011, the Company has reflected $1.3 million of investment income and related compensation expense of $0.2 million within the broker-dealer segment in proportion to its capital.

[6] Economic Income (Loss) recognizes revenues (i) net of distribution fees paid to agents and (ii) our proportionate share of management and incentive fees of certain real estate operating entities and the activist business (2012 only).

[7] Economic Income (Loss) recognizes Company income from proprietary trading net of related expenses.

[8] Economic Income (Loss) excludes income taxes as management does not consider this item when evaluating the performance of the segment. Also, reimbursement from affiliates is shown as a reduction of Economic Income expenses, but is included as a part of revenues under US GAAP.

[9] Economic Income (Loss) recognizes the Company's proportionate share of expenses for certain real estate and other operating entities for which the investments are recorded under the equity method of accounting for investments.

[10] Economic Income (Loss) excludes discontinued operations.