Segment Reporting (Details)		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) segment customer	Sep. 30, 2014 USD ($) customer	Dec. 31, 2013 customer
Segment Reporting Information [Line Items]
Number of Operating Segments | segment				2
Revenues
Investment banking		$ 53,012,000	$ 45,799,000	$ 186,763,000	$ 125,653,000
Brokerage		41,085,000	34,334,000	111,496,000	100,475,000
Management fees		10,519,000	9,796,000	31,169,000	28,412,000
Incentive income		93,000	(837,000)	365,000	4,385,000
Investment Income		0	0	0	0
Interest and dividends		3,604,000	13,725,000	9,846,000	35,437,000
Reimbursement from affiliates		3,355,000	2,473,000	10,499,000	7,391,000
Other revenue		1,312,000	1,120,000	2,684,000	2,427,000
Total revenues		113,254,000	107,101,000	353,956,000	306,680,000
Expenses
Non-interest expense		92,966,000	99,636,000	335,902,000	295,603,000
Interest and dividends		7,367,000	12,429,000	19,241,000	29,694,000
Total expenses		100,967,000	112,700,000	356,769,000	326,632,000
Other income (loss)
Total other income (loss)		(23,370,000)	16,269,000	32,073,000	58,372,000
Income tax expense (benefit)		(5,081,000)	141,000	5,212,000	266,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(4,344,000)	(4,006,000)	(10,980,000)	(13,409,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(10,346,000)	6,523,000	$ 13,068,000	$ 24,745,000
Number of major customers | customer				0	0	0
Reported Under Economic Income / (Loss)
Revenues
Investment banking		53,012,000	45,799,000	$ 186,763,000	$ 125,653,000
Brokerage		41,875,000	35,583,000	112,333,000	104,983,000
Management fees		18,021,000	16,319,000	51,167,000	46,574,000
Incentive income		(8,629,000)	4,488,000	(1,081,000)	16,900,000
Investment Income		(22,024,000)	7,463,000	19,072,000	37,232,000
Interest and dividends		0	0	0	0
Reimbursement from affiliates		0	0	0	0
Other revenue		564,000	396,000	689,000	494,000
Total revenues		82,819,000	110,048,000	368,943,000	331,836,000
Expenses
Non-interest expense		90,854,000	98,804,000	331,103,000	292,334,000
Interest and dividends		4,257,000	2,664,000	12,425,000	5,961,000
Total expenses		95,111,000	101,468,000	343,528,000	298,295,000
Other income (loss)
Total other income (loss)		0	0	0	0
Income tax expense (benefit)		0	0	0	0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(2,231,000)	(1,877,000)	(6,128,000)	(8,320,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(14,523,000)	6,703,000	19,287,000	25,221,000
Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Investment banking		0	0	0	0
Brokerage		18,000	15,000	41,000	40,000
Management fees		17,739,000	16,319,000	50,885,000	46,574,000
Incentive income		(8,629,000)	4,488,000	(1,081,000)	16,900,000
Investment Income		(16,781,000)	4,064,000	14,309,000	29,025,000
Interest and dividends		0	0	0	0
Reimbursement from affiliates		0	0	0	0
Other revenue		(9,000)	43,000	84,000	140,000
Total revenues		(7,662,000)	24,929,000	64,238,000	92,679,000
Expenses
Non-interest expense		6,133,000	21,754,000	57,164,000	71,421,000
Interest and dividends		2,970,000	1,852,000	8,880,000	5,083,000
Total expenses		9,103,000	23,606,000	66,044,000	76,504,000
Other income (loss)
Total other income (loss)		0	0	0	0
Income tax expense (benefit)		0	0	0	0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(2,231,000)	(1,877,000)	(6,128,000)	(8,320,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(18,996,000)	(554,000)	(7,934,000)	7,855,000
Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Investment banking		53,012,000	45,799,000	186,763,000	125,653,000
Brokerage		41,857,000	35,568,000	112,292,000	104,943,000
Management fees		282,000	0	282,000	0
Incentive income		0	0	0	0
Investment Income		(5,243,000)	3,399,000	4,763,000	8,207,000
Interest and dividends		0	0	0	0
Reimbursement from affiliates		0	0	0	0
Other revenue		573,000	353,000	605,000	354,000
Total revenues		90,481,000	85,119,000	304,705,000	239,157,000
Expenses
Non-interest expense		84,721,000	77,050,000	273,939,000	220,913,000
Interest and dividends		1,287,000	812,000	3,545,000	878,000
Total expenses		86,008,000	77,862,000	277,484,000	221,791,000
Other income (loss)
Total other income (loss)		0	0	0	0
Income tax expense (benefit)		0	0	0	0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		0	0	0	0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		4,473,000	7,257,000	27,221,000	17,366,000
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		(371,000)	(246,000)	(937,000)	(723,000)
Incentive income		29,000	(34,000)	(278,000)	(188,000)
Investment Income		0	0	0	0
Interest and dividends		0	0	0	0
Reimbursement from affiliates		(85,000)	119,000	(261,000)	(45,000)
Other revenue		0	0	0	0
Total revenues		(153,000)	530,000	(342,000)	1,544,000
Expenses
Non-interest expense		0	0	0	0
Interest and dividends		0	0	0	0
Total expenses		634,000	635,000	1,626,000	1,335,000
Other income (loss)
Total other income (loss)		554,000	888,000	4,278,000	3,509,000
Income tax expense (benefit)		0	0	0	0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		233,000	(783,000)	(2,310,000)	(3,718,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0	0	0	0
Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking		0	0	0	0
Brokerage	[1]	(790,000)	(1,249,000)	(837,000)	(4,508,000)
Management fees	[2]	(7,131,000)	(6,277,000)	(19,061,000)	(17,439,000)
Incentive income	[2]	8,693,000	(5,291,000)	1,724,000	(12,327,000)
Investment Income	[3]	22,024,000	(7,463,000)	(19,072,000)	(37,232,000)
Interest and dividends	[1],[3]	3,604,000	13,725,000	9,846,000	35,437,000
Reimbursement from affiliates	[4]	3,440,000	2,354,000	10,760,000	7,436,000
Other revenue	[3]	748,000	724,000	1,995,000	1,933,000
Total revenues		30,588,000	(3,477,000)	(14,645,000)	(26,700,000)
Expenses
Non-interest expense	[3],[5]	2,112,000	832,000	4,799,000	3,269,000
Interest and dividends	[1],[3]	3,110,000	9,765,000	6,816,000	23,733,000
Total expenses		5,222,000	10,597,000	11,615,000	27,002,000
Other income (loss)
Total other income (loss)	[3]	(23,924,000)	15,381,000	27,795,000	54,863,000
Income tax expense (benefit)	[6]	(5,081,000)	141,000	5,212,000	266,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(2,346,000)	(1,346,000)	(2,542,000)	(1,371,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		4,177,000	(180,000)	(6,219,000)	(476,000)
Consolidated Funds
Revenues
Total revenues		274,000	691,000	1,134,000	2,500,000
Expenses
Total expenses		634,000	635,000	1,626,000	1,335,000
Consolidated Funds | Reported Under Economic Income / (Loss)
Revenues
Total revenues		0	0	0	0
Expenses
Total expenses		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
Consolidated Funds | Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Total revenues		0	0	0	0
Expenses
Total expenses		0	0	0	0
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Total revenues		274,000	691,000	1,134,000	2,500,000
Expenses
Total expenses		634,000	635,000	1,626,000	1,335,000
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

[1] Economic Income (Loss) recognizes stock borrow/loan activity (prior to January 2015) and other brokerage dividends as brokerage revenue.

[2] 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.

[3] Economic Income (Loss) recognizes Company income from proprietary trading (including interest and dividends) net of related expenses.

[4] Reimbursement from affiliates is shown as a reduction of Economic Income expenses, but is included as a part of revenues under US GAAP.

[5] 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.

[6] Economic Income (Loss) excludes income taxes as management does not consider this item when evaluating the performance of the segment.