Segment Reporting (Details)		3 Months Ended			6 Months Ended
		Jun. 30, 2016 USD ($)		Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment customer		Jun. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer					0
Number of Operating Segments | segment					2
Revenues
Investment banking		$ 35,287,000		$ 68,518,000	$ 61,434,000		$ 133,751,000
Brokerage		47,100,000		34,957,000	98,035,000		70,411,000
Management fees		10,649,000		10,266,000	21,679,000		20,650,000
Incentive income		428,000		(2,100,000)	1,539,000		272,000
Investment Income		0		0	0		0
Interest and dividends		4,105,000		3,159,000	7,758,000		6,242,000
Aircraft lease revenue		1,982,000		0	1,982,000		0
Reimbursement from affiliates		2,241,000		3,502,000	6,128,000		7,144,000
Other revenue		13,346,000		704,000	16,071,000		1,372,000
Total revenues		117,231,000		119,608,000	218,270,000		240,702,000
Expenses
Non-interest expense		108,639,000		110,267,000	215,452,000		242,936,000
Interest and dividends		6,944,000		6,095,000	14,254,000		11,874,000
Total expenses		117,726,000		116,996,000	233,665,000		255,802,000
Other income (loss)
Total other income (loss)		(38,658,000)		11,366,000	(35,073,000)		55,443,000
Income tax expense (benefit)		(11,992,000)		3,346,000	(15,312,000)		10,293,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		16,705,000		(3,916,000)	21,002,000		(6,636,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(10,456,000)		6,716,000	(14,154,000)		23,414,000
Consolidated Funds
Revenues
Total revenues		2,093,000		602,000	3,644,000		860,000
Expenses
Total expenses		2,143,000		634,000	3,959,000		992,000
Operating Segments
Revenues
Investment banking		35,287,000		68,518,000	61,434,000		133,751,000
Brokerage		49,026,000		34,934,000	101,893,000		70,458,000
Management fees		16,495,000		16,540,000	33,399,000		33,147,000
Incentive income		429,000		(7,815,000)	7,349,000		7,547,000
Investment Income		(22,171,000)		12,215,000	(20,322,000)		41,095,000
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		0		0	0		0
Other revenue		1,328,000		57,000	2,281,000		125,000
Total revenues		80,394,000		124,449,000	186,034,000		286,123,000
Expenses
Non-interest expense		96,019,000		109,088,000	200,042,000		240,250,000
Interest and dividends		4,375,000		4,145,000	8,649,000		8,167,000
Total expenses		100,394,000		113,233,000	208,691,000		248,417,000
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		(2,008,000)		(1,051,000)	(4,235,000)		(3,896,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(22,008,000)		10,165,000	(26,892,000)		33,810,000
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		5,000	0		23,000
Management fees		15,729,000		16,540,000	31,858,000		33,147,000
Incentive income		429,000		(7,815,000)	7,349,000		7,547,000
Investment Income		(16,636,000)		9,259,000	(15,242,000)		31,090,000
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		0		0	0		0
Other revenue		1,540,000		72,000	2,222,000		93,000
Total revenues		1,062,000		18,061,000	26,187,000		71,900,000
Expenses
Non-interest expense		14,067,000		16,198,000	34,329,000		51,032,000
Interest and dividends		3,257,000		2,866,000	6,440,000		5,910,000
Total expenses		17,324,000		19,064,000	40,769,000		56,942,000
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		(2,008,000)		(1,051,000)	(4,235,000)		(3,896,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(18,270,000)		(2,054,000)	(18,817,000)		11,062,000
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		35,287,000		68,518,000	61,434,000		133,751,000
Brokerage		49,026,000		34,929,000	101,893,000		70,435,000
Management fees		766,000		0	1,541,000		0
Incentive income		0		0	0		0
Investment Income		(5,535,000)		2,956,000	(5,080,000)		10,005,000
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		0		0	0		0
Other revenue		(212,000)		(15,000)	59,000		32,000
Total revenues		79,332,000		106,388,000	159,847,000		214,223,000
Expenses
Non-interest expense		81,952,000		92,890,000	165,713,000		189,218,000
Interest and dividends		1,118,000		1,279,000	2,209,000		2,257,000
Total expenses		83,070,000		94,169,000	167,922,000		191,475,000
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.		(3,738,000)		12,219,000	(8,075,000)		22,748,000
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		(424,000)		(330,000)	(794,000)		(567,000)
Incentive income		150,000		(124,000)	(7,000)		(306,000)
Investment Income		0		0	0		0
Interest and dividends		0		0	0		0
Aircraft lease revenue		0			0
Reimbursement from affiliates		(134,000)		(91,000)	(148,000)		(176,000)
Other revenue		0		0	0		0
Total revenues		1,685,000		57,000	2,695,000		(189,000)
Expenses
Non-interest expense		0		0	0		0
Interest and dividends		0		0	0		0
Total expenses		2,143,000		634,000	3,959,000		992,000
Other income (loss)
Total other income (loss)		(17,771,000)		1,317,000	(21,184,000)		3,724,000
Income tax expense (benefit)		0		0	0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		18,229,000		(740,000)	22,448,000		(2,543,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0	0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		2,093,000		602,000	3,644,000		860,000
Expenses
Total expenses		2,143,000		634,000	3,959,000		992,000
Adjustments | Other Adjustments
Revenues
Investment banking		0		0	0		0
Brokerage	[1]	(1,926,000)		23,000	(3,858,000)		(47,000)
Management fees	[2]	(5,422,000)		(5,944,000)	(10,926,000)		(11,930,000)
Incentive income	[2]	(151,000)		5,839,000	(5,803,000)		(6,969,000)
Investment Income	[3]	22,171,000		(12,215,000)	20,322,000		(41,095,000)
Interest and dividends	[3]	4,105,000		3,159,000	7,758,000		6,242,000
Aircraft lease revenue	[4]	1,982,000			1,982,000
Reimbursement from affiliates	[1]	2,375,000		3,593,000	6,276,000		7,320,000
Other revenue	[3]	12,019,000	[4]	647,000	13,790,000	[4]	1,247,000
Total revenues		35,153,000		(4,898,000)	29,541,000		(45,232,000)
Expenses
Non-interest expense	[3],[5],[6]	12,620,000		1,179,000	15,410,000		2,686,000
Interest and dividends	[3]	2,569,000		1,950,000	5,605,000		3,707,000
Total expenses		15,189,000		3,129,000	21,015,000		6,393,000
Other income (loss)
Total other income (loss)	[3]	(20,888,000)		10,049,000	(13,889,000)		51,719,000
Income tax expense (benefit)	[5]	(11,992,000)		3,346,000	(15,312,000)		10,293,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		484,000		(2,125,000)	2,789,000		(197,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		11,552,000		(3,449,000)	12,738,000		(10,396,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0	0		0
Expenses
Total expenses		$ 0		$ 0	$ 0		$ 0

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

[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) and underwriting income from the Company's insurance related activities net of expenses.

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

[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.