Segment Reporting (Details) (USD $)	3 Months Ended					9 Months Ended
	Sep. 30, 2014 customer		Sep. 30, 2013		Mar. 31, 2013 customer	Sep. 30, 2014 segment customer		Sep. 30, 2013
Segment Reporting Information [Line Items]
Number of Operating Segments						2
Revenues
Investment banking	$ 45,799,000		$ 27,694,000			$ 125,653,000		$ 70,431,000
Brokerage	34,334,000		28,462,000			100,475,000		86,583,000
Management fees	9,796,000		9,327,000			28,412,000		28,518,000
Incentive income	(837,000)		2,521,000			4,385,000		7,086,000
Investment Income	0		0			0		0
Interest and dividends	13,725,000		10,969,000			35,437,000		30,905,000
Reimbursement from affiliates	2,473,000		2,175,000			7,391,000		6,451,000
Other revenue	1,120,000		556,000			2,427,000		1,519,000
Total revenues	107,101,000		82,149,000			306,680,000		232,174,000
Expenses
Employee compensation and benefits	65,806,000		53,614,000			197,771,000		145,344,000
Non-compensation expenses - Fixed	0		0			0		0
Non-compensation expenses - Variable	0		0			0		0
Non-compensation expenses	33,830,000		31,392,000			97,832,000		93,994,000
Interest and dividends	12,429,000		6,281,000			29,694,000		20,886,000
Reimbursement from affiliates	0		0			0		0
Total expenses	112,700,000		91,737,000			326,632,000		261,461,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	13,836,000		14,680,000			48,227,000		30,507,000
Total other income (loss)	16,269,000		17,912,000			58,372,000		42,212,000
Income (loss) before income taxes and non-controlling interests	10,670,000		8,324,000			38,420,000		12,925,000
Income tax expense (benefit)	141,000		(46,000)			266,000		288,000
Economic income (loss)/ net income (loss) before non-controlling interests	10,529,000		8,370,000			38,154,000		12,637,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(4,006,000)		(4,759,000)			(13,409,000)		(10,509,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	6,523,000		3,611,000			24,745,000		2,128,000
Number of major customers	0				0	0
Reported Under Economic Income/(Loss)
Revenues
Investment banking	45,799,000		27,694,000			125,653,000		70,431,000
Brokerage	35,583,000		32,035,000			104,983,000		93,352,000
Management fees	16,319,000		14,299,000			46,574,000		43,050,000
Incentive income	4,746,000		5,666,000			17,665,000		14,558,000
Investment Income	7,463,000		12,327,000			37,232,000		26,762,000
Interest and dividends	0		0			0		0
Reimbursement from affiliates	0		0			0		0
Other revenue	396,000		118,000			494,000		(46,000)
Total revenues	110,306,000		92,139,000			332,601,000		248,107,000
Expenses
Employee compensation and benefits	64,968,000		52,842,000			195,557,000		143,702,000
Non-compensation expenses - Fixed	23,767,000		23,774,000			69,596,000		67,972,000
Non-compensation expenses - Variable	12,157,000		8,430,000			33,259,000		27,706,000
Non-compensation expenses	0		0			0		0
Interest and dividends	2,664,000		75,000			5,961,000		271,000
Reimbursement from affiliates	(1,830,000)		(1,461,000)			(5,313,000)		(4,291,000)
Total expenses	101,726,000		83,660,000			299,060,000		235,360,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Total other income (loss)	0		0			0		0
Income (loss) before income taxes and non-controlling interests	8,580,000		8,479,000			33,541,000		12,747,000
Income tax expense (benefit)	0		0			0		0
Economic income (loss)/ net income (loss) before non-controlling interests	8,580,000		8,479,000			33,541,000		12,747,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,877,000)		(4,813,000)			(8,320,000)		(8,880,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	6,703,000		3,666,000			25,221,000		3,867,000
Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Investment banking	0		0			0		0
Brokerage	15,000		0			40,000		0
Management fees	16,319,000		14,299,000			46,574,000		43,050,000
Incentive income	4,746,000		5,666,000			17,665,000		14,558,000
Investment Income	4,064,000		10,031,000			29,025,000		22,169,000
Interest and dividends	0		0			0		0
Reimbursement from affiliates	0		0			0		0
Other revenue	43,000		156,000			140,000		381,000
Total revenues	25,187,000		30,152,000			93,444,000		80,158,000
Expenses
Employee compensation and benefits	11,307,000		12,551,000			43,360,000		35,500,000
Non-compensation expenses - Fixed	8,887,000		8,938,000			27,944,000		26,293,000
Non-compensation expenses - Variable	3,648,000		1,597,000			6,195,000		3,996,000
Non-compensation expenses	0		0			0		0
Interest and dividends	1,852,000		52,000			5,083,000		183,000
Reimbursement from affiliates	(1,830,000)		(1,461,000)			(5,313,000)		(4,291,000)
Total expenses	23,864,000		21,677,000			77,269,000		61,681,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Total other income (loss)	0		0			0		0
Income (loss) before income taxes and non-controlling interests	1,323,000		8,475,000			16,175,000		18,477,000
Income tax expense (benefit)	0		0			0		0
Economic income (loss)/ net income (loss) before non-controlling interests	1,323,000		8,475,000			16,175,000		18,477,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,877,000)		(4,813,000)			(8,320,000)		(8,880,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	(554,000)		3,662,000			7,855,000		9,597,000
Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Investment banking	45,799,000	[1]	27,694,000	[2]		125,653,000	[3]	70,431,000	[4]
Brokerage	35,568,000	[1]	32,035,000	[2]		104,943,000	[3]	93,352,000	[4]
Management fees	0	[1]	0	[2]		0	[3]	0	[4]
Incentive income	0	[1]	0	[2]		0	[3]	0	[4]
Investment Income	3,399,000	[1]	2,296,000	[2]		8,207,000	[3]	4,593,000	[4]
Interest and dividends	0	[1]	0	[2]		0	[3]	0	[4]
Reimbursement from affiliates	0	[1]	0	[2]		0	[3]	0	[4]
Other revenue	353,000	[1]	(38,000)	[2]		354,000	[3]	(427,000)	[4]
Total revenues	85,119,000	[1]	61,987,000	[2]		239,157,000	[3]	167,949,000	[4]
Expenses
Employee compensation and benefits	53,661,000	[1]	40,291,000	[2]		152,197,000	[4]	108,202,000	[4]
Non-compensation expenses - Fixed	14,880,000	[1]	14,836,000	[2]		41,652,000	[4]	41,679,000	[4]
Non-compensation expenses - Variable	8,509,000	[1]	6,833,000	[2]		27,064,000	[4]	23,710,000	[4]
Non-compensation expenses	0	[1]	0	[2]		0	[4]	0	[4]
Interest and dividends	812,000	[1]	23,000	[2]		878,000	[4]	88,000	[4]
Reimbursement from affiliates	0	[1]	0	[2]		0	[4]	0	[4]
Total expenses	77,862,000	[1]	61,983,000	[2]		221,791,000	[4]	173,679,000	[4]
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0	[1]	0	[2]		0	[3]	0	[4]
Total other income (loss)	0	[1]	0	[2]		0	[3]	0	[4]
Income (loss) before income taxes and non-controlling interests	7,257,000	[1]	4,000	[2]		17,366,000	[3]	(5,730,000)	[4]
Income tax expense (benefit)	0	[1]	0	[2]		0	[3]	0	[4]
Economic income (loss)/ net income (loss) before non-controlling interests	7,257,000	[1]	4,000	[2]		17,366,000	[3]	(5,730,000)	[4]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	0	[1]	0	[2]		0	[3]	0	[4]
Net income (loss) attributable to Cowen Group, Inc. stockholders	7,257,000	[1]	4,000	[2]		17,366,000	[3]	(5,730,000)	[4]
Investment Income from Entity's Capital	1,900,000		3,000,000			7,500,000		5,600,000
Compensation Expense Related to Investment of Entity's Capital	700,000		1,000,000			2,500,000		1,800,000
Significant Reconciling Items | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking	0		0			0		0
Brokerage	(1,249,000)	[5]	(3,573,000)	[5]		(4,508,000)	[5]	(6,769,000)	[5]
Management fees	(6,277,000)	[6]	(4,697,000)	[6]		(17,439,000)	[6]	(13,661,000)	[6]
Incentive income	(5,549,000)	[6]	(3,145,000)	[6]		(13,092,000)	[6]	(7,472,000)	[6]
Investment Income	(7,463,000)	[7]	(12,327,000)	[7]		(37,232,000)	[7]	(26,762,000)	[7]
Interest and dividends	13,725,000	[5],[7]	10,969,000	[5],[7]		35,437,000	[5],[7]	30,905,000	[5],[7]
Reimbursement from affiliates	2,354,000	[8]	2,250,000	[8]		7,436,000	[8]	6,657,000	[8]
Other revenue	724,000	[7]	438,000	[7]		1,933,000	[7]	1,565,000	[7]
Total revenues	(3,735,000)		(10,085,000)			(27,465,000)		(15,537,000)
Expenses
Employee compensation and benefits	838,000		772,000			2,214,000		1,642,000
Non-compensation expenses - Fixed	(23,767,000)	[7],[9]	(23,774,000)	[7],[9]		(69,596,000)	[7],[9]	(67,972,000)	[7],[9]
Non-compensation expenses - Variable	(12,157,000)	[7],[9]	(8,430,000)	[7],[9]		(33,259,000)	[7],[9]	(27,706,000)	[7],[9]
Non-compensation expenses	33,830,000	[7],[9]	31,392,000	[7],[9]		97,832,000	[7],[9]	93,994,000	[7],[9]
Interest and dividends	9,765,000	[5],[7]	6,206,000	[5],[7]		23,733,000	[5],[7]	20,615,000	[5],[7]
Reimbursement from affiliates	1,830,000	[8]	1,461,000	[8]		5,313,000	[8]	4,291,000	[8]
Total expenses	10,339,000		7,627,000			26,237,000		24,864,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	13,836,000	[7]	14,680,000	[7]		48,227,000	[7]	30,507,000	[7]
Total other income (loss)	15,381,000		16,682,000			54,863,000		37,350,000
Income (loss) before income taxes and non-controlling interests	1,307,000		(1,030,000)			1,161,000		(3,051,000)
Income tax expense (benefit)	141,000	[10]	(46,000)	[10]		266,000	[10]	288,000	[10]
Economic income (loss)/ net income (loss) before non-controlling interests	1,166,000		(984,000)			895,000		(3,339,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,346,000)		929,000			(1,371,000)		1,600,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	(180,000)		(55,000)			(476,000)		(1,739,000)
Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking	0		0			0		0
Brokerage	0		0			0		0
Management fees	(246,000)		(275,000)			(723,000)		(871,000)
Incentive income	(34,000)		0			(188,000)		0
Investment Income	0		0			0		0
Interest and dividends	0		0			0		0
Reimbursement from affiliates	119,000		(75,000)			(45,000)		(206,000)
Other revenue	0		0			0		0
Total revenues	530,000		95,000			1,544,000		(396,000)
Expenses
Employee compensation and benefits	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
Interest and dividends	0		0			0		0
Reimbursement from affiliates	0		0			0		0
Total expenses	635,000		450,000			1,335,000		1,237,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Total other income (loss)	888,000		1,230,000			3,509,000		4,862,000
Income (loss) before income taxes and non-controlling interests	783,000		875,000			3,718,000		3,229,000
Income tax expense (benefit)	0		0			0		0
Economic income (loss)/ net income (loss) before non-controlling interests	783,000		875,000			3,718,000		3,229,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(783,000)		(875,000)			(3,718,000)		(3,229,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	0		0			0		0
Consolidated Funds
Revenues
Total revenues	691,000		445,000			2,500,000		681,000
Expenses
Total expenses	635,000		450,000			1,335,000		1,237,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	2,433,000		3,232,000			10,145,000		11,705,000
Consolidated Funds | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0		0			0		0
Expenses
Total expenses	0		0			0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Consolidated Funds | Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0		0			0		0
Expenses
Total expenses	0		0			0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Consolidated Funds | Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0	[1]	0	[2]		0	[3]	0	[4]
Expenses
Total expenses	0	[1]	0	[2]		0	[4]	0	[4]
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0	[1]	0	[2]		0	[3]	0	[4]
Consolidated Funds | Significant Reconciling Items | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues	0		0			0		0
Expenses
Total expenses	0		0			0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	1,545,000		2,002,000			6,636,000		6,843,000
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues	691,000		445,000			2,500,000		681,000
Expenses
Total expenses	635,000		450,000			1,335,000		1,237,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	$ 888,000		$ 1,230,000			$ 3,509,000		$ 4,862,000

[1] For the three months ended September 30, 2014, the Company has reflected $1.9 million of investment income and related compensation expense of $0.7 million within the broker-dealer segment in proportion to that segment's capital.

[2] For the three months ended September 30, 2013, the Company has reflected $3.0 million of investment income and related compensation expense of $1.0 million within the broker-dealer segment in proportion to that segment's capital.

[3] For the nine months ended September 30, 2014, the Company has reflected $7.5 million of investment income and related compensation expense of $2.5 million within the broker-dealer segment in proportion to that segment's capital.

[4] For the nine months ended September 30, 2013, 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 that segment's capital.

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

[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.

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

[8] 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 income taxes as management does not consider this item when evaluating the performance of the segment.