Segment Reporting (Details)		3 Months Ended			9 Months Ended
		Sep. 30, 2016 USD ($)		Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment customer		Sep. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer					0
Number of Operating Segments | segment					2
Revenues
Investment banking		$ 36,722,000		$ 53,012,000	$ 98,156,000		$ 186,763,000
Brokerage		49,605,000		41,085,000	147,640,000		111,496,000
Management fees		10,272,000		10,519,000	31,951,000		31,169,000
Incentive income		1,284,000		93,000	2,823,000		365,000
Investment Income		0		0	0		0
Interest and dividends		3,906,000		3,604,000	11,664,000		9,846,000
Aircraft lease revenue		1,089,000		0	3,071,000		0
Reimbursement from affiliates		2,140,000		3,355,000	8,268,000		10,499,000
Other revenue		25,112,000		1,312,000	41,183,000		2,684,000
Total revenues		131,027,000		113,254,000	349,297,000		353,956,000
Expenses
Non-interest expense		148,178,000		92,966,000	363,630,000		335,902,000
Interest and dividends		7,612,000		7,367,000	21,866,000		19,241,000
Total expenses		158,259,000		100,967,000	391,924,000		356,769,000
Other income (loss)
Total other income (loss)		51,250,000		(23,370,000)	16,177,000		32,073,000
Income tax expense (benefit)		8,759,000		(5,081,000)	(6,553,000)		5,212,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(18,478,000)		(4,344,000)	2,524,000		(10,980,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(3,219,000)		(10,346,000)	(17,373,000)		13,068,000
Consolidated Funds
Revenues
Total revenues		897,000		274,000	4,541,000		1,134,000
Expenses
Total expenses		2,469,000		634,000	6,428,000		1,626,000
Operating Segments
Revenues
Investment banking		36,722,000		53,012,000	98,156,000		186,763,000
Brokerage		51,524,000		41,875,000	153,417,000		112,333,000
Management fees		16,539,000		18,021,000	49,938,000		51,167,000
Incentive income		11,825,000		(8,629,000)	19,174,000		(1,081,000)
Investment Income		19,730,000		(22,024,000)	(591,000)		19,072,000
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		0		0	0		0
Other revenue		17,170,000		564,000	19,450,000		689,000
Total revenues		153,510,000		82,819,000	339,544,000		368,943,000
Expenses
Non-interest expense		135,848,000		90,854,000	335,889,000		331,103,000
Interest and dividends		4,284,000		4,257,000	12,933,000		12,425,000
Total expenses		140,132,000		95,111,000	348,822,000		343,528,000
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		(2,354,000)		(2,231,000)	(6,589,000)		(6,128,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		11,024,000		(14,523,000)	(15,867,000)		19,287,000
Operating Segments | Consolidated Funds
Revenues
Total revenues		0		0	0		0
Expenses
Total expenses		0		0	0		0
Operating Segments | Alternative Investment
Revenues
Investment banking		0		0	0		0
Brokerage		0		18,000	0		41,000
Management fees		15,746,000		17,739,000	47,604,000		50,885,000
Incentive income		11,825,000		(8,629,000)	19,174,000		(1,081,000)
Investment Income		14,798,000		(16,781,000)	(444,000)		14,309,000
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		0		0	0		0
Other revenue		17,145,000		(9,000)	19,366,000		84,000
Total revenues		59,514,000		(7,662,000)	85,700,000		64,238,000
Expenses
Non-interest expense		43,589,000		6,133,000	77,917,000		57,164,000
Interest and dividends		3,205,000		2,970,000	9,645,000		8,880,000
Total expenses		46,794,000		9,103,000	87,562,000		66,044,000
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		(2,354,000)		(2,231,000)	(6,589,000)		(6,128,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		10,366,000		(18,996,000)	(8,451,000)		(7,934,000)
Operating Segments | Alternative Investment | Consolidated Funds
Revenues
Total revenues		0		0	0		0
Expenses
Total expenses		0		0	0		0
Operating Segments | Broker-Dealer
Revenues
Investment banking		36,722,000		53,012,000	98,156,000		186,763,000
Brokerage		51,524,000		41,857,000	153,417,000		112,292,000
Management fees		793,000		282,000	2,334,000		282,000
Incentive income		0		0	0		0
Investment Income		4,932,000		(5,243,000)	(147,000)		4,763,000
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		0		0	0		0
Other revenue		25,000		573,000	84,000		605,000
Total revenues		93,996,000		90,481,000	253,844,000		304,705,000
Expenses
Non-interest expense		92,259,000		84,721,000	257,972,000		273,939,000
Interest and dividends		1,079,000		1,287,000	3,288,000		3,545,000
Total expenses		93,338,000		86,008,000	261,260,000		277,484,000
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.		658,000		4,473,000	(7,416,000)		27,221,000
Operating Segments | Broker-Dealer | Consolidated Funds
Revenues
Total revenues		0		0	0		0
Expenses
Total expenses		0		0	0		0
Adjustments | Funds Consolidation
Revenues
Investment banking		0		0	0		0
Brokerage		0		0	0		0
Management fees		(445,000)		(371,000)	(1,239,000)		(937,000)
Incentive income		(562,000)		29,000	(569,000)		(278,000)
Investment Income		0		0	0		0
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		(76,000)		(85,000)	(224,000)		(261,000)
Other revenue		0		0	0		0
Total revenues		(186,000)		(153,000)	2,509,000		(342,000)
Expenses
Non-interest expense		(429,000)		0	(429,000)		0
Interest and dividends		0		0	0		0
Total expenses		2,040,000		634,000	5,999,000		1,626,000
Other income (loss)
Total other income (loss)		19,269,000		554,000	(1,915,000)		4,278,000
Income tax expense (benefit)		0		0	0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(17,043,000)		233,000	5,405,000		(2,310,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0	0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		897,000		274,000	4,541,000		1,134,000
Expenses
Total expenses		2,469,000		634,000	6,428,000		1,626,000
Adjustments | Other Adjustments
Revenues
Investment banking		0		0	0		0
Brokerage		(1,919,000)		(790,000)	(5,777,000)		(837,000)
Management fees	[1]	(5,822,000)		(7,131,000)	(16,748,000)		(19,061,000)
Incentive income	[1]	(9,979,000)		8,693,000	(15,782,000)		1,724,000
Investment Income	[2]	(19,730,000)		22,024,000	591,000		(19,072,000)
Interest and dividends	[2]	3,906,000		3,604,000	11,664,000		9,846,000
Aircraft lease revenue	[3]	1,089,000			3,071,000
Reimbursement from affiliates	[4]	2,216,000		3,440,000	8,492,000		10,760,000
Other revenue	[2]	7,942,000	[3]	748,000	21,733,000	[3]	1,995,000
Total revenues		(22,297,000)		30,588,000	7,244,000		(14,645,000)
Expenses
Non-interest expense	[2],[5],[6]	12,759,000		2,112,000	28,170,000		4,799,000
Interest and dividends	[2]	3,328,000		3,110,000	8,933,000		6,816,000
Total expenses		16,087,000		5,222,000	37,103,000		11,615,000
Other income (loss)
Total other income (loss)	[2]	31,981,000		(23,924,000)	18,092,000		27,795,000
Income tax expense (benefit)	[5]	8,759,000		(5,081,000)	(6,553,000)		5,212,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		919,000		(2,346,000)	3,708,000		(2,542,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(14,243,000)		4,177,000	(1,506,000)		(6,219,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0	0		0
Expenses
Total expenses		$ 0		$ 0	$ 0		$ 0

[1] 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.

[2] Economic Income (Loss) recognizes Company income from proprietary trading (including interest and dividends) and underwriting income from the Company's insurance related activities net of expenses.

[3] Aircraft lease revenue is shown net of expenses in other revenue for Economic Income (Loss).

[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) excludes income taxes and acquisition related adjustments as management does not consider these items when evaluating the performance of the segment.

[6] 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.