Segment Reporting (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2012 segment		Jun. 30, 2011		Jun. 30, 2012 segment		Jun. 30, 2011
Segment Reporting Information [Line Items]
Number of business segments, in segments	2				2
Revenues
Investment banking	$ 16,254,000		$ 14,343,000		$ 31,884,000		$ 29,025,000
Brokerage	24,568,000		24,607,000		48,581,000		52,198,000
Management fees	9,932,000		11,857,000		19,649,000		23,021,000
Incentive income	580,000		675,000		1,271,000		5,056,000
Interest and dividends	5,868,000		5,840,000		11,240,000		10,399,000
Reimbursement from affiliates	1,381,000		981,000		2,426,000		1,990,000
Other revenue	831,000		232,000		1,698,000		922,000
Expenses
Employee compensation and benefits	43,097,000		43,575,000		89,780,000		88,662,000
Interest and dividends	3,207,000		3,115,000		4,931,000		5,724,000
Total expenses	77,425,000		81,228,000		154,164,000		163,458,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	9,787,000		76,000		29,458,000		17,358,000
Total other income (loss)	7,766,000		27,090,000		33,404,000		46,118,000
Income taxes expense / (benefit)	191,000		(17,954,000)		333,000		(17,791,000)
Economic Income (Loss) / Net income (loss) before non-controlling interests	(10,380,000)		22,495,000		(4,144,000)		23,375,000
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(7,946,000)		20,037,000		(3,951,000)		20,119,000
Reported Under U.S. GAAP [Member]
Revenues
Investment banking	16,254,000		14,343,000		31,884,000		29,025,000
Brokerage	24,568,000		24,607,000		48,581,000		52,198,000
Management fees	9,932,000		11,857,000		19,649,000		23,021,000
Incentive income	580,000		675,000		1,271,000		5,056,000
Investment Income	0		0		0		0
Interest and dividends	5,868,000		5,840,000		11,240,000		10,399,000
Reimbursement from affiliates	1,381,000		981,000		2,426,000		1,990,000
Other revenue	831,000		232,000		1,698,000		922,000
Revenues	59,470,000		58,679,000		116,949,000		122,924,000
Expenses
Employee compensation and benefits	43,097,000		43,575,000		89,780,000		88,662,000
Interest and dividends	3,207,000		3,115,000		4,931,000		5,724,000
Non-compensation expenses - Fixed	0		0		0		0
Non-compensation expenses - Variable	0		0		0		0
Non-compensation expenses	30,486,000		33,666,000		58,444,000		67,571,000
Reimbursement from affiliates	0		0		0		0
Total expenses	77,425,000		81,228,000		154,164,000		163,458,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	9,787,000		76,000		29,458,000		17,358,000
Bargain purchase gain			22,244,000	[1]			22,244,000	[1]
Total other income (loss)	7,766,000		27,090,000		33,404,000		46,118,000
Income (loss) before income taxes and non-controlling interests	(10,189,000)		4,541,000		(3,811,000)		5,584,000
Income taxes expense / (benefit)	191,000		(17,954,000)		333,000		(17,791,000)
Economic Income (Loss) / Net income (loss) before non-controlling interests	(10,380,000)		22,495,000		(4,144,000)		23,375,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	2,434,000		(2,458,000)		193,000		(3,256,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(7,946,000)		20,037,000		(3,951,000)		20,119,000
Operating Segments [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking	16,254,000		14,343,000		31,884,000		29,025,000
Brokerage	24,568,000		24,607,000		48,581,000		52,198,000
Management fees	14,586,000		15,539,000		28,606,000		29,586,000
Incentive income	2,583,000		5,697,000		6,605,000		10,860,000
Investment Income	8,286,000		22,700,000		29,391,000		39,909,000
Interest and dividends	0		0		0		0
Reimbursement from affiliates	0		0		0		0
Other revenue	(71,000)		(481,000)		313,000		573,000
Revenues	66,206,000		82,405,000		145,380,000		162,151,000
Expenses
Employee compensation and benefits	41,645,000		42,367,000		87,554,000		85,104,000
Interest and dividends	59,000		218,000		147,000		435,000
Non-compensation expenses - Fixed	24,489,000		26,276,000		45,732,000		47,863,000
Non-compensation expenses - Variable	7,138,000		12,506,000		14,013,000		21,454,000
Non-compensation expenses	0		0		0		0
Reimbursement from affiliates	(1,435,000)		(989,000)		(2,551,000)		(2,158,000)
Total expenses	71,896,000		80,378,000		144,895,000		152,698,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Bargain purchase gain			0	[1]			0	[1]
Total other income (loss)	0		0		0		0
Income (loss) before income taxes and non-controlling interests	(5,690,000)		2,027,000		485,000		9,453,000
Income taxes expense / (benefit)	0		0		0		0
Economic Income (Loss) / Net income (loss) before non-controlling interests	(5,690,000)		2,027,000		485,000		9,453,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(300,000)		(1,465,000)		(600,000)		(1,940,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(5,990,000)		562,000		(115,000)		7,513,000
Alternative Investment [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking	0		0		0		0
Brokerage	0		11,000		0		74,000
Management fees	14,586,000		15,539,000		28,606,000		29,586,000
Incentive income	2,583,000		5,697,000		6,605,000		10,860,000
Investment Income	6,694,000		22,789,000		23,496,000		33,822,000
Interest and dividends	0		0		0		0
Reimbursement from affiliates	0		0		0		0
Other revenue	216,000		(278,000)		340,000		974,000
Revenues	24,079,000		43,758,000		59,047,000		75,316,000
Expenses
Employee compensation and benefits	13,500,000		15,037,000		30,946,000		30,871,000
Interest and dividends	12,000		49,000		30,000		105,000
Non-compensation expenses - Fixed	8,560,000		8,724,000		15,529,000		15,718,000
Non-compensation expenses - Variable	1,138,000		6,591,000		2,447,000		8,687,000
Non-compensation expenses	0		0		0		0
Reimbursement from affiliates	(1,435,000)		(989,000)		(2,551,000)		(2,158,000)
Total expenses	21,775,000		29,412,000		46,401,000		53,223,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Bargain purchase gain			0	[1]			0	[1]
Total other income (loss)	0		0		0		0
Income (loss) before income taxes and non-controlling interests	2,304,000		14,346,000		12,646,000		22,093,000
Income taxes expense / (benefit)	0		0		0		0
Economic Income (Loss) / Net income (loss) before non-controlling interests	2,304,000		14,346,000		12,646,000		22,093,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(300,000)		(1,465,000)		(600,000)		(1,940,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	2,004,000		12,881,000		12,046,000		20,153,000
Broker-Dealer [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking	16,254,000	[2]	14,343,000	[3]	31,884,000	[4]	29,025,000	[5]
Brokerage	24,568,000	[2]	24,596,000	[3]	48,581,000	[4]	52,124,000	[5]
Management fees	0	[2]	0	[3]	0	[4]	0	[5]
Incentive income	0	[2]	0	[3]	0	[4]	0	[5]
Investment Income	1,592,000	[2]	(89,000)	[3]	5,895,000	[4]	6,087,000	[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	(287,000)	[2]	(203,000)	[3]	(27,000)	[4]	(401,000)	[5]
Revenues	42,127,000	[2]	38,647,000	[3]	86,333,000	[4]	86,835,000	[5]
Expenses
Employee compensation and benefits	28,145,000	[2]	27,330,000	[3]	56,608,000	[4]	54,233,000	[5]
Interest and dividends	47,000	[2]	169,000	[3]	117,000	[4]	330,000	[5]
Non-compensation expenses - Fixed	15,929,000	[2]	17,552,000	[3]	30,203,000	[4]	32,145,000	[5]
Non-compensation expenses - Variable	6,000,000	[2]	5,915,000	[3]	11,566,000	[4]	12,767,000	[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	50,121,000	[2]	50,966,000	[3]	98,494,000	[4]	99,475,000	[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],[3]			0	[1],[5]
Total other income (loss)	0	[2]	0	[3]	0	[4]	0	[5]
Income (loss) before income taxes and non-controlling interests	(7,994,000)	[2]	(12,319,000)	[3]	(12,161,000)	[4]	(12,640,000)	[5]
Income taxes expense / (benefit)	0	[2]	0	[3]	0	[4]	0	[5]
Economic Income (Loss) / Net income (loss) before non-controlling interests	(7,994,000)	[2]	(12,319,000)	[3]	(12,161,000)	[4]	(12,640,000)	[5]
(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	(7,994,000)	[2]	(12,319,000)	[3]	(12,161,000)	[4]	(12,640,000)	[5]
Investment Income from Entity's Capital	1,700,000		6,400,000		5,900,000		6,400,000
Compensation Expense Related to Investment of Entity's Capital	600,000		1,900,000		1,900,000		1,900,000
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	(394,000)		(466,000)		(787,000)		(979,000)
Incentive income	0		0		0		0
Investment Income	0		0		0		0
Interest and dividends	0		0		0		0
Reimbursement from affiliates	(54,000)		(8,000)		(125,000)		(168,000)
Other revenue	0		0		0		0
Revenues	(392,000)		(330,000)		(712,000)		(834,000)
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	635,000		872,000		1,009,000		1,501,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0		0		0
Bargain purchase gain			0	[1]			0	[1]
Total other income (loss)	(1,707,000)		2,195,000		934,000		3,651,000
Income (loss) before income taxes and non-controlling interests	(2,734,000)		993,000		(787,000)		1,316,000
Income taxes expense / (benefit)	0		0		0		0
Economic Income (Loss) / Net income (loss) before non-controlling interests	(2,734,000)		993,000		(787,000)		1,316,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	2,734,000		(993,000)		787,000		(1,316,000)
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	(4,260,000)	[6]	(3,216,000)	[6]	(8,170,000)	[6]	(5,586,000)	[6]
Incentive income	(2,003,000)	[6]	(5,022,000)	[6]	(5,334,000)	[6]	(5,804,000)	[6]
Investment Income	(8,286,000)	[7]	(22,700,000)	[7]	(29,391,000)	[7]	(39,909,000)	[7]
Interest and dividends	5,868,000	[7]	5,840,000	[7]	11,240,000	[7]	10,399,000	[7]
Reimbursement from affiliates	1,435,000	[8]	989,000	[8]	2,551,000	[8]	2,158,000	[8]
Other revenue	902,000	[7]	713,000	[7]	1,385,000	[7]	349,000	[7]
Revenues	(6,344,000)		(23,396,000)		(27,719,000)		(38,393,000)
Expenses
Employee compensation and benefits	1,452,000		1,208,000		2,226,000		3,558,000
Interest and dividends	3,148,000	[7]	2,897,000	[7]	4,784,000	[7]	5,289,000	[7]
Non-compensation expenses - Fixed	(24,489,000)	[7],[9]	(26,276,000)	[7],[9]	(45,732,000)	[7],[9]	(47,863,000)	[7],[9]
Non-compensation expenses - Variable	(7,138,000)	[7],[9]	(12,506,000)	[7],[9]	(14,013,000)	[7],[9]	(21,454,000)	[7],[9]
Non-compensation expenses	30,486,000	[7],[9]	33,666,000	[7],[9]	58,444,000	[7],[9]	67,571,000	[7],[9]
Reimbursement from affiliates	1,435,000	[8]	989,000	[8]	2,551,000	[8]	2,158,000	[8]
Total expenses	4,894,000		(22,000)		8,260,000		9,259,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	9,787,000	[7]	76,000	[7]	29,458,000	[7]	17,358,000	[7]
Bargain purchase gain			22,244,000	[1]			22,244,000	[1]
Total other income (loss)	9,473,000		24,895,000		32,470,000		42,467,000
Income (loss) before income taxes and non-controlling interests	(1,765,000)		1,521,000		(3,509,000)		(5,185,000)
Income taxes expense / (benefit)	191,000	[8]	(17,954,000)	[8]	333,000	[8]	(17,791,000)	[8]
Economic Income (Loss) / Net income (loss) before non-controlling interests	(1,956,000)		19,475,000		(3,842,000)		12,606,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	0		0		6,000		0
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(1,956,000)		19,475,000		(3,836,000)		12,606,000
Consolidated Funds [Member] | Reported Under U.S. GAAP [Member]
Revenues
Revenues	56,000		144,000		200,000		313,000
Expenses
Total expenses	635,000		872,000		1,009,000		1,501,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	(2,021,000)		4,770,000		3,946,000		6,516,000
Consolidated Funds [Member] | Operating Segments [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
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
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
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
Revenues	56,000		144,000		200,000		313,000
Expenses
Total expenses	635,000		872,000		1,009,000		1,501,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	(1,707,000)		2,195,000		934,000		3,651,000
Consolidated Funds [Member] | Other Adjustments [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Revenues	0		0		0		0
Expenses
Total expenses	0		0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	$ (314,000)		$ 2,575,000		$ 3,012,000		$ 2,865,000

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

[2] For the three months ended June 30, 2012 the Company has reflected $1.7 million of investment income and related compensation expense of $0.6 million within the broker-dealer segment in proportion to its capital.

[3] For the three months ended June 30, 2011, the Company has reflected $6.4 million of investment income and related compensation expense of $1.9 million within the broker-dealer segment in proportion to its capital.

[4] For the six months ended June 30, 2012, the Company has reflected $5.9 million of investment income and related compensation expense of $1.9 million within the broker-dealer segment in proportion to its capital.

[5] For the six months ended June 30, 2011, the Company has reflected $6.4 million of investment income and related compensation expense of $1.9 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.