Segment Reporting (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013 customer	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012 customer	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 customer segment		Dec. 31, 2012 customer		Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of Operating Segments									2
Revenues
Investment banking									$ 105,333,000		$ 71,762,000		$ 50,976,000
Brokerage									114,593,000		91,167,000		99,611,000
Management fees									37,303,000		38,116,000		52,466,000
Incentive income									12,586,000		5,411,000		3,265,000
Interest and dividends									39,454,000		24,608,000		22,306,000
Reimbursement from affiliates									9,161,000		5,239,000		4,322,000
Total revenues	97,438,000	81,360,000	81,207,000	67,241,000	65,932,000	57,598,000	59,470,000	57,480,000	327,246,000		240,480,000		235,278,000
Expenses
Employee compensation and benefits									207,248,000		194,034,000		203,767,000
Interest and dividends									27,299,000		12,137,000		9,233,000
Goodwill impairment									0		0		7,151,000
Total expenses									360,917,000		326,900,000		375,655,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									40,924,000		55,665,000		15,128,000
Bargain purchase gain									0		0		22,244,000
Total other income (loss)									51,968,000		62,911,000		41,767,000
Income (loss) before income taxes and non-controlling interests	5,372,000	8,324,000	3,506,000	1,095,000	(8,243,000)	(11,455,000)	(10,189,000)	6,378,000	18,297,000		(23,509,000)		(98,610,000)
Income tax expense (benefit)	169,000	(46,000)	158,000	176,000	(48,000)	163,000	191,000	142,000	457,000		448,000		(20,073,000)
Economic income (loss)/ net income (loss) before non-controlling interests									17,840,000		(23,957,000)		(102,183,000)
Net income (loss) from discontinued operations, net of tax									0		0		(23,646,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	2,519,000	3,611,000	1,093,000	(2,576,000)	(9,349,000)	(10,585,000)	(7,946,000)	3,995,000	4,647,000		(23,885,000)		(108,010,000)
Number of major customers	0				0				0		0
Reported Under U.S. GAAP
Revenues
Investment banking									105,333,000		71,762,000		50,976,000
Brokerage									114,593,000		91,167,000		99,611,000
Management fees									37,303,000		38,116,000		52,466,000
Incentive income									12,586,000		5,411,000		3,265,000
Investment Income									0		0		0
Interest and dividends									39,454,000		24,608,000		22,306,000
Reimbursement from affiliates									9,161,000		5,239,000		4,322,000
Other revenue									5,418,000		3,668,000		1,583,000
Total revenues									327,246,000		240,480,000		235,278,000
Expenses
Employee compensation and benefits									207,248,000		194,034,000		203,767,000
Interest and dividends									27,299,000		12,137,000		9,233,000
Non-compensation expenses - Fixed									0		0		0
Non-compensation expenses - Variable									0		0		0
Non-compensation expenses									124,331,000		119,053,000		152,722,000
Goodwill impairment													7,151,000
Reimbursement from affiliates									0		0		0
Total expenses									360,917,000		326,900,000		375,655,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									40,924,000		55,665,000		15,128,000
Bargain purchase gain													22,244,000
Total other income (loss)									51,968,000		62,911,000		41,767,000
Income (loss) before income taxes and non-controlling interests									18,297,000		(23,509,000)		(98,610,000)
Income tax expense (benefit)									457,000		448,000		(20,073,000)
Economic income (loss)/ net income (loss) before non-controlling interests									17,840,000		(23,957,000)		(78,537,000)
Net income (loss) from discontinued operations, net of tax													(23,646,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(13,193,000)		72,000		(5,827,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders									4,647,000		(23,885,000)		(108,010,000)
Operating Segments | Reported Under Economic Income/(Loss)
Revenues
Investment banking									105,333,000		71,762,000		50,976,000
Brokerage									121,065,000		93,903,000		99,611,000
Management fees									56,984,000		56,381,000		67,309,000
Incentive income									21,933,000		15,205,000		10,366,000
Investment Income									36,660,000		50,116,000		41,347,000
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									2,534,000		1,248,000		615,000
Total revenues									344,509,000		288,615,000		270,224,000
Expenses
Employee compensation and benefits									204,836,000		188,155,000		192,558,000
Interest and dividends									350,000		339,000		735,000
Non-compensation expenses - Fixed									95,524,000		95,462,000		103,181,000
Non-compensation expenses - Variable									30,643,000		25,275,000		41,497,000
Non-compensation expenses									0		0		0
Goodwill impairment													0
Reimbursement from affiliates									(6,380,000)		(5,527,000)		(4,602,000)
Total expenses									324,973,000		303,704,000		333,369,000
Other income (loss)
Net gain (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									19,536,000		(15,089,000)		(63,145,000)
Income tax expense (benefit)									0		0		0
Economic income (loss)/ net income (loss) before non-controlling interests									19,536,000		(15,089,000)		(63,145,000)
Net income (loss) from discontinued operations, net of tax													0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(12,995,000)		(2,480,000)		(8,292,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders									6,541,000		(17,569,000)		(71,437,000)
Alternative Investment
Expenses
Goodwill impairment									0		0
Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Investment banking									0		0		0
Brokerage									0		0		0
Management fees									56,984,000		56,381,000		67,309,000
Incentive income									21,933,000		15,205,000		10,366,000
Investment Income									30,713,000		40,374,000		33,599,000
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									524,000		844,000		622,000
Total revenues									110,154,000		112,804,000		111,896,000
Expenses
Employee compensation and benefits									54,656,000		59,647,000		46,757,000
Interest and dividends									231,000		151,000		184,000
Non-compensation expenses - Fixed									33,986,000		32,575,000		33,954,000
Non-compensation expenses - Variable									4,520,000		4,941,000		17,085,000
Non-compensation expenses									0		0		0
Goodwill impairment													0
Reimbursement from affiliates									(6,380,000)		(5,527,000)		(4,602,000)
Total expenses									87,013,000		91,787,000		93,378,000
Other income (loss)
Net gain (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									23,141,000		21,017,000		18,518,000
Income tax expense (benefit)									0		0		0
Economic income (loss)/ net income (loss) before non-controlling interests									23,141,000		21,017,000		18,518,000
Net income (loss) from discontinued operations, net of tax													0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(12,995,000)		(2,480,000)		(8,292,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders									10,146,000		18,537,000		10,226,000
Broker-Dealer
Expenses
Goodwill impairment									0		0		7,200,000
Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Investment banking									105,333,000	[1]	71,762,000	[2]	50,976,000	[3]
Brokerage									121,065,000	[1]	93,903,000	[2]	99,611,000	[3]
Management fees									0	[1]	0	[2]	0	[3]
Incentive income									0	[1]	0	[2]	0	[3]
Investment Income									5,947,000	[1]	9,742,000	[2]	7,748,000	[3]
Interest and dividends									0	[1]	0	[2]	0	[3]
Reimbursement from affiliates									0	[1]	0	[2]	0	[3]
Other revenue									2,010,000	[1]	404,000	[2]	(7,000)	[3]
Total revenues									234,355,000	[1]	175,811,000	[2]	158,328,000	[3]
Expenses
Employee compensation and benefits									150,180,000	[1]	128,508,000	[2]	145,801,000	[3]
Interest and dividends									119,000	[1]	188,000	[2]	551,000	[3]
Non-compensation expenses - Fixed									61,538,000	[1]	62,887,000	[2]	69,227,000	[3]
Non-compensation expenses - Variable									26,123,000	[1]	20,334,000	[2]	24,412,000	[3]
Non-compensation expenses									0	[1]	0	[2]	0	[3]
Goodwill impairment													0
Reimbursement from affiliates									0	[1]	0	[2]	0	[3]
Total expenses									237,960,000	[1]	211,917,000	[2]	239,991,000	[3]
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									0	[1]	0	[2]	0	[3]
Bargain purchase gain													0	[3]
Total other income (loss)									0	[1]	0	[2]	0	[3]
Income (loss) before income taxes and non-controlling interests									(3,605,000)	[1]	(36,106,000)	[2]	(81,663,000)	[3]
Income tax expense (benefit)									0	[1]	0	[2]	0	[3]
Economic income (loss)/ net income (loss) before non-controlling interests									(3,605,000)	[1]	(36,106,000)	[2]	(81,663,000)	[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]
Net income (loss) attributable to Cowen Group, Inc. stockholders									(3,605,000)	[1]	(36,106,000)	[2]	(81,663,000)	[3]
Investment Income from Entity's Capital									7,600,000		10,200,000		5,600,000
Compensation Expense Related to Investment of Entity's Capital									2,500,000		3,400,000		1,800,000
Significant Reconciling Items | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking									0		0		0
Brokerage									(6,472,000)	[4]	(2,736,000)	[4]	0
Management fees									(18,535,000)	[5]	(16,791,000)	[5]	(13,034,000)	[5]
Incentive income									(9,347,000)	[5]	(9,794,000)	[5]	(7,101,000)	[5]
Investment Income									(36,660,000)	[6]	(50,116,000)	[6]	(41,347,000)	[6]
Interest and dividends									39,454,000	[6]	24,608,000	[6]	22,306,000	[6]
Reimbursement from affiliates									9,260,000	[7]	5,527,000	[7]	4,602,000	[7]
Other revenue									2,884,000	[6]	2,420,000	[6]	968,000	[6]
Total revenues									(19,416,000)		(46,882,000)		(33,606,000)
Expenses
Employee compensation and benefits									2,412,000		5,879,000		11,209,000
Interest and dividends									26,949,000	[6]	11,798,000	[6]	8,498,000	[6]
Non-compensation expenses - Fixed									(95,524,000)	[6],[8]	(95,462,000)	[6],[8]	(103,181,000)	[6],[8]
Non-compensation expenses - Variable									(30,643,000)	[6],[8]	(25,275,000)	[6],[8]	(41,497,000)	[6],[8]
Non-compensation expenses									124,331,000	[6],[8]	119,053,000	[6],[8]	152,722,000	[6],[8]
Goodwill impairment													7,151,000	[9]
Reimbursement from affiliates									6,380,000	[7]	5,527,000	[7]	4,602,000	[7]
Total expenses									33,905,000		21,520,000		39,504,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									40,924,000	[6]	55,665,000	[6]	15,128,000	[6]
Bargain purchase gain													22,244,000	[10]
Total other income (loss)									49,350,000		60,355,000		38,820,000
Income (loss) before income taxes and non-controlling interests									(3,971,000)		(8,047,000)		(34,290,000)
Income tax expense (benefit)									457,000	[7]	448,000	[7]	(20,073,000)	[7]
Economic income (loss)/ net income (loss) before non-controlling interests									(4,428,000)		(8,495,000)		(14,217,000)
Net income (loss) from discontinued operations, net of tax													(23,646,000)	[11]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									2,534,000		2,179,000		1,290,000
Net income (loss) attributable to Cowen Group, Inc. stockholders									(1,894,000)		(6,316,000)		(36,573,000)
Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking									0		0		0
Brokerage									0		0		0
Management fees									(1,146,000)		(1,474,000)		(1,809,000)
Incentive income									0		0		0
Investment Income									0		0		0
Interest and dividends									0		0		0
Reimbursement from affiliates									(99,000)		(288,000)		(280,000)
Other revenue									0		0		0
Total revenues									2,153,000		(1,253,000)		(1,340,000)
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													0
Reimbursement from affiliates									0		0		0
Total expenses									2,039,000		1,676,000		2,782,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									0		0		0
Bargain purchase gain													0
Total other income (loss)									2,618,000		2,556,000		2,947,000
Income (loss) before income taxes and non-controlling interests									2,732,000		(373,000)		(1,175,000)
Income tax expense (benefit)									0		0		0
Economic income (loss)/ net income (loss) before non-controlling interests									2,732,000		(373,000)		(1,175,000)
Net income (loss) from discontinued operations, net of tax													0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(2,732,000)		373,000		1,175,000
Net income (loss) attributable to Cowen Group, Inc. stockholders									0		0		0
Consolidated Funds | Reported Under U.S. GAAP
Revenues
Total revenues									3,398,000		509,000		749,000
Expenses
Total expenses									2,039,000		1,676,000		2,782,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									11,044,000		7,246,000		4,395,000
Consolidated Funds | Operating Segments | Reported Under Economic Income/(Loss)
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									0		0		0
Consolidated Funds | Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									0		0		0
Consolidated Funds | Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Total revenues									0	[1]	0	[2]	0	[3]
Expenses
Total expenses									0	[1]	0	[2]	0	[3]
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									0	[1]	0	[2]	0	[3]
Consolidated Funds | Significant Reconciling Items | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues									0				0
Expenses
Total expenses									0				0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									8,426,000				1,448,000
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues									3,398,000		509,000		749,000
Expenses
Total expenses									2,039,000		1,676,000		2,782,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments									2,618,000		2,556,000		2,947,000
Consolidated Funds | Other Adjustments [Member] | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues											0
Expenses
Total expenses											0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments											$ 4,690,000

[1] For the year ended December 31, 2013, the Company has reflected $7.6 million of investment income and related compensation expense of $2.5 million within the broker-dealer segment in proportion to its capital.

[2] 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.

[3] 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.

[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.

[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.