Segment Reporting (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012 segment	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 segment		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Number of business segments, in segments	2								2
Revenues
Investment banking									$ 71,762,000		$ 50,976,000		$ 38,965,000
Brokerage									91,167,000		99,611,000		112,217,000
Management fees									38,116,000		52,466,000		38,847,000
Incentive income									5,411,000		3,265,000		11,363,000
Interest and dividends									24,608,000		22,306,000		11,547,000
Reimbursement from affiliates									5,239,000		4,322,000		6,816,000
Other revenue									3,668,000		1,583,000		1,936,000
Expenses
Employee compensation and benefits									194,034,000		203,767,000		194,919,000
Interest and dividends									11,760,000		8,839,000		8,971,000
Goodwill, Impairment Loss									0		7,151,000		0
Total expenses									326,900,000		375,655,000		339,942,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									55,665,000		15,128,000		21,980,000
Total other income (loss)									62,911,000		41,767,000		53,042,000
Income taxes expense / (benefit)	(48,000)	163,000	191,000	142,000	(2,353,000)	71,000	(17,954,000)	163,000	448,000		(20,073,000)		(21,400,000)
Net income (loss)									(23,957,000)		(102,183,000)		(31,690,000)
Net income (loss) from discontinued operations, net of tax	0	0	0	0	(18,559,000)	(5,087,000)	0	0	0		(23,646,000)		0
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders	(9,349,000)	(10,585,000)	(7,946,000)	3,995,000	(79,896,000)	(48,233,000)	20,037,000	82,000	(23,885,000)		(108,010,000)		(45,417,000)
Reported Under U.S. GAAP [Member]
Revenues
Investment banking									71,762,000		50,976,000		38,965,000
Brokerage									91,167,000		99,611,000		112,217,000
Management fees									38,116,000		52,466,000		38,847,000
Incentive income									5,411,000		3,265,000		11,363,000
Investment Income									0		0		0
Interest and dividends									24,608,000		22,306,000		11,547,000
Reimbursement from affiliates									5,239,000		4,322,000		6,816,000
Other revenue									3,668,000		1,583,000		1,936,000
Total revenues									240,480,000		235,278,000		233,810,000
Expenses
Employee compensation and benefits									194,034,000		203,767,000		194,919,000
Interest and dividends									11,760,000		8,839,000		8,971,000
Non-compensation expenses - Fixed									0		0		0
Non-compensation expenses - Variable									0		0		0
Non-compensation expenses									119,430,000		153,116,000		127,931,000
Goodwill, Impairment Loss											7,151,000
Reimbursement from affiliates									0		0		0
Total expenses									326,900,000		375,655,000		339,942,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									55,665,000		15,128,000		21,980,000
Bargain purchase gain											22,244,000
Total other income (loss)									62,911,000		41,767,000		53,042,000
Income (loss) before income taxes and non-controlling interests									(23,509,000)		(98,610,000)		(53,090,000)
Income taxes expense / (benefit)									448,000		(20,073,000)		(21,400,000)
Net income (loss)									(23,957,000)		(78,537,000)		(31,690,000)
Net income (loss) from discontinued operations, net of tax											(23,646,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									72,000		(5,827,000)		(13,727,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders									(23,885,000)		(108,010,000)		(45,417,000)
Operating Segments [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking									71,762,000		50,976,000		38,965,000
Brokerage									93,903,000		99,611,000		112,217,000
Management fees									56,381,000		67,309,000		51,440,000
Incentive income									15,205,000		10,366,000		9,615,000
Investment Income									50,116,000		41,347,000		59,417,000
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									1,248,000		615,000		939,000
Total revenues									288,615,000		270,224,000		272,593,000
Expenses
Employee compensation and benefits									190,405,000		194,808,000		185,893,000
Interest and dividends									339,000		735,000		1,026,000
Non-compensation expenses - Fixed									95,462,000		103,181,000		92,457,000
Non-compensation expenses - Variable									25,275,000		41,497,000		34,360,000
Non-compensation expenses									0		0		0
Goodwill, Impairment Loss											0
Reimbursement from affiliates									(5,527,000)		(4,602,000)		(7,315,000)
Total expenses									305,954,000		335,619,000		306,421,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0		0
Bargain purchase gain											0
Total other income (loss)									0		0		0
Income (loss) before income taxes and non-controlling interests									(17,339,000)		(65,395,000)		(33,828,000)
Income taxes expense / (benefit)									0		0		0
Net income (loss)									(17,339,000)		(65,395,000)		(33,828,000)
Net income (loss) from discontinued operations, net of tax											0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(230,000)		(6,042,000)		(1,759,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders									(17,569,000)		(71,437,000)		(35,587,000)
Alternative Investment [Member]
Expenses
Goodwill, Impairment Loss									0		0
Alternative Investment [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking									0		0		0
Brokerage									0		0		0
Management fees									56,381,000		67,309,000		51,440,000
Incentive income									15,205,000		10,366,000		9,615,000
Investment Income									40,374,000		33,599,000		50,958,000
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									844,000		622,000		932,000
Total revenues									112,804,000		111,896,000		112,945,000
Expenses
Employee compensation and benefits									61,897,000		49,007,000		55,966,000
Interest and dividends									151,000		184,000		265,000
Non-compensation expenses - Fixed									32,575,000		33,954,000		28,963,000
Non-compensation expenses - Variable									4,941,000		17,085,000		7,338,000
Non-compensation expenses									0		0		0
Goodwill, Impairment Loss											0
Reimbursement from affiliates									(5,527,000)		(4,602,000)		(7,315,000)
Total expenses									94,037,000		95,628,000		85,217,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0		0
Bargain purchase gain											0
Total other income (loss)									0		0		0
Income (loss) before income taxes and non-controlling interests									18,767,000		16,268,000		27,728,000
Income taxes expense / (benefit)									0		0		0
Net income (loss)									18,767,000		16,268,000		27,728,000
Net income (loss) from discontinued operations, net of tax											0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(230,000)		(6,042,000)		(1,759,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders									18,537,000		10,226,000		25,969,000
Broker-Dealer [Member]
Expenses
Goodwill, Impairment Loss									0		7,151,000
Broker-Dealer [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Investment banking									71,762,000	[1]	50,976,000	[2]	38,965,000	[3]
Brokerage									93,903,000	[1]	99,611,000	[2]	112,217,000	[3]
Management fees									0	[1]	0	[2]	0	[3]
Incentive income									0	[1]	0	[2]	0	[3]
Investment Income									9,742,000	[1]	7,748,000	[2]	8,459,000	[3]
Interest and dividends									0	[1]	0	[2]	0	[3]
Reimbursement from affiliates									0	[1]	0	[2]	0	[3]
Other revenue									404,000	[1]	(7,000)	[2]	7,000	[3]
Total revenues									175,811,000	[1]	158,328,000	[2]	159,648,000	[3]
Expenses
Employee compensation and benefits									128,508,000	[1]	145,801,000	[2]	129,927,000	[3]
Interest and dividends									188,000	[1]	551,000	[2]	761,000	[3]
Non-compensation expenses - Fixed									62,887,000	[1]	69,227,000	[2]	63,494,000	[3]
Non-compensation expenses - Variable									20,334,000	[1]	24,412,000	[2]	27,022,000	[3]
Non-compensation expenses									0	[1]	0	[2]	0	[3]
Goodwill, Impairment Loss											0	[2]
Reimbursement from affiliates									0	[1]	0	[2]	0	[3]
Total expenses									211,917,000	[1]	239,991,000	[2]	221,204,000	[3]
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0	[2]	0	[3]
Bargain purchase gain											0	[2]
Total other income (loss)									0	[1]	0	[2]	0	[3]
Income (loss) before income taxes and non-controlling interests									(36,106,000)	[1]	(81,663,000)	[2]	(61,556,000)	[3]
Income taxes expense / (benefit)									0	[1]	0	[2]	0	[3]
Net income (loss)									(36,106,000)	[1]	(81,663,000)	[2]	(61,556,000)	[3]
Net income (loss) from discontinued operations, net of tax											0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									0	[1]	0	[2]	0	[3]
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders									(36,106,000)	[1]	(81,663,000)	[2]	(61,556,000)	[3]
Investment Income from Entity's Capital									10,200,000		5,600,000		8,700,000
Compensation Expense Related to Investment of Entity's Capital									3,400,000		1,800,000		2,900,000
Funds Consolidation [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Investment banking									0		0		0
Brokerage									0		0		0
Management fees									(1,474,000)		(1,809,000)		(2,877,000)
Incentive income									0		0		0
Investment Income									0		0		0
Interest and dividends									0		0		0
Reimbursement from affiliates									(288,000)		(280,000)		(499,000)
Other revenue									0		0		0
Total revenues									(1,253,000)		(1,340,000)		8,743,000
Expenses
Employee compensation and benefits									0		0		0
Interest and dividends									0		0		0
Non-compensation expenses - Fixed									0		0		0
Non-compensation expenses - Variable									0		0		0
Non-compensation expenses									0		0		0
Goodwill, Impairment Loss											0
Reimbursement from affiliates									0		0		0
Total expenses									1,676,000		2,782,000		8,121,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0		0
Bargain purchase gain											0
Total other income (loss)									2,556,000		2,947,000		11,346,000
Income (loss) before income taxes and non-controlling interests									(373,000)		(1,175,000)		11,968,000
Income taxes expense / (benefit)									0		0		0
Net income (loss)									(373,000)		(1,175,000)		11,968,000
Net income (loss) from discontinued operations, net of tax											0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									373,000		1,175,000		(11,968,000)
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders									0		0		0
Other Adjustments [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Investment banking									0		0		0
Brokerage									(2,736,000)	[4]	0		0
Management fees									(16,791,000)	[5]	(13,034,000)	[5]	(9,716,000)	[5]
Incentive income									(9,794,000)	[5]	(7,101,000)	[5]	1,748,000	[5]
Investment Income									(50,116,000)	[6]	(41,347,000)	[6]	(59,417,000)	[6]
Interest and dividends									24,608,000	[6]	22,306,000	[6]	11,547,000	[6]
Reimbursement from affiliates									5,527,000	[7]	4,602,000	[7]	7,315,000	[7]
Other revenue									2,420,000	[6]	968,000	[6]	997,000	[6]
Total revenues									(46,882,000)		(33,606,000)		(47,526,000)
Expenses
Employee compensation and benefits									3,629,000		8,959,000		9,026,000
Interest and dividends									11,421,000	[6]	8,104,000	[6]	7,945,000	[6]
Non-compensation expenses - Fixed									(95,462,000)	[6],[8]	(103,181,000)	[6],[8]	(92,457,000)	[6],[8]
Non-compensation expenses - Variable									(25,275,000)	[6],[8]	(41,497,000)	[6],[8]	(34,360,000)	[6],[8]
Non-compensation expenses									119,430,000	[6],[8]	153,116,000	[6],[8]	127,931,000	[6],[8]
Goodwill, Impairment Loss											7,151,000	[9]
Reimbursement from affiliates									5,527,000	[7]	4,602,000	[7]	7,315,000	[7]
Total expenses									19,270,000		37,254,000		25,400,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									55,665,000	[6]	15,128,000	[6]	21,980,000	[6]
Bargain purchase gain											22,244,000	[10]
Total other income (loss)									60,355,000		38,820,000		41,696,000
Income (loss) before income taxes and non-controlling interests									(5,797,000)		(32,040,000)		(31,230,000)
Income taxes expense / (benefit)									448,000	[7]	(20,073,000)	[7]	(21,400,000)	[7]
Net income (loss)									(6,245,000)		(11,967,000)		(9,830,000)
Net income (loss) from discontinued operations, net of tax											(23,646,000)	[11]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(71,000)		(960,000)		0
Economic Income (Loss) / Net Income (loss) attributable to Cowen Group, Inc. stockholders									(6,316,000)		(36,573,000)		(9,830,000)
Consolidated Funds | Reported Under U.S. GAAP [Member]
Revenues
Total revenues									509,000		749,000		12,119,000
Expenses
Total expenses									1,676,000		2,782,000		8,121,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									7,246,000		4,395,000		31,062,000
Consolidated Funds | Operating Segments [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0		0
Consolidated Funds | Alternative Investment [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0		0
Consolidated Funds | Broker-Dealer [Member] | Reported Under Economic Income/(Loss) [Member]
Revenues
Total revenues									0	[1]	0	[2]	0	[3]
Expenses
Total expenses									0	[1]	0	[2]	0	[3]
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									0		0	[2]	0	[3]
Consolidated Funds | Funds Consolidation [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Total revenues									509,000		749,000		12,119,000
Expenses
Total expenses									1,676,000		2,782,000		8,121,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									2,556,000		2,947,000		11,346,000
Consolidated Funds | Other Adjustments [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP [Member]
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments									$ 4,690,000		$ 1,448,000		$ 19,716,000

[1] For the year ended December 31, 2012, the Company has reflected $10.2 million of investment income and related compensation expense of $3.4 million within the broker-dealer segment in proportion to its capital.

[2] For the year ended December 31, 2011, the Company has reflected $5.6 million of investment income and related compensation expense of $1.8 million within the broker-dealer segment in proportion to its capital.

[3] For the year ended December 31, 2010, the Company has reflected $8.7 million of investment income and related compensation expense of $2.9 million within the broker-dealer segment in proportion to its capital.

[4] Economic Income (Loss) recognizes stock borrow/loan activity and other brokerage dividends as brokerage revenue.

[5] 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 and 2011 only).

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

[7] 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.

[8] 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.

[9] Economic Income (Loss) excludes goodwill impairment.

[10] Economic Income (Loss) excludes the bargain purchase gain which resulted from the LaBranche acquisition.

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