Segment Reporting (Details)		3 Months Ended
		Mar. 31, 2016 USD ($) segment customer		Mar. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer		0
Number of Operating Segments | segment		2
Revenues
Investment banking		$ 26,147,000		$ 65,233,000
Brokerage		50,935,000		35,454,000
Management fees		11,030,000		10,384,000
Incentive income		1,111,000		2,372,000
Investment Income		0		0
Interest and dividends		3,653,000		3,083,000
Reimbursement from affiliates		3,887,000		3,642,000
Other revenue		2,725,000		668,000
Total revenues		101,039,000		121,094,000
Expenses
Non-interest expense		106,813,000		132,669,000
Interest and dividends		7,310,000		5,779,000
Total expenses		115,939,000		138,806,000
Other income (loss)
Total other income (loss)		3,585,000		44,077,000
Income tax expense (benefit)		(3,320,000)		6,947,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		4,297,000		(2,720,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(3,698,000)		16,698,000
Reported Under Economic Income / (Loss)
Revenues
Investment banking		26,147,000		65,233,000
Brokerage		52,867,000		35,523,000
Management fees		16,903,000		16,607,000
Incentive income		6,920,000		15,363,000
Investment Income		1,850,000		28,880,000
Interest and dividends		0		0
Reimbursement from affiliates		0		0
Other revenue		953,000		69,000
Total revenues		105,640,000		161,675,000
Expenses
Non-interest expense		104,022,000		131,163,000
Interest and dividends		4,273,000		4,022,000
Total expenses		108,295,000		135,185,000
Other income (loss)
Total other income (loss)		0		0
Income tax expense (benefit)		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(2,227,000)		(2,845,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(4,882,000)		23,645,000
Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Investment banking		0		0
Brokerage		0		18,000
Management fees		16,128,000		16,607,000
Incentive income		6,920,000		15,363,000
Investment Income		1,395,000		21,830,000
Interest and dividends		0		0
Reimbursement from affiliates		0		0
Other revenue		682,000		21,000
Total revenues		25,125,000		53,839,000
Expenses
Non-interest expense		20,261,000		34,834,000
Interest and dividends		3,183,000		3,044,000
Total expenses		23,444,000		37,878,000
Other income (loss)
Total other income (loss)		0		0
Income tax expense (benefit)		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(2,227,000)		(2,845,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(546,000)		13,116,000
Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Investment banking		26,147,000		65,233,000
Brokerage		52,867,000		35,505,000
Management fees		775,000		0
Incentive income		0		0
Investment Income		455,000		7,050,000
Interest and dividends		0		0
Reimbursement from affiliates		0		0
Other revenue		271,000		48,000
Total revenues		80,515,000		107,836,000
Expenses
Non-interest expense		83,761,000		96,329,000
Interest and dividends		1,090,000		978,000
Total expenses		84,851,000		97,307,000
Other income (loss)
Total other income (loss)		0		0
Income tax expense (benefit)		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		0		0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(4,336,000)		10,529,000
Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking		0		0
Brokerage		0		0
Management fees		(370,000)		(238,000)
Incentive income		(157,000)		(183,000)
Investment Income		0		0
Interest and dividends		0		0
Reimbursement from affiliates		(14,000)		(85,000)
Other revenue		0		0
Total revenues		1,010,000		(248,000)
Expenses
Non-interest expense		0		0
Interest and dividends		0		0
Total expenses		1,816,000		358,000
Other income (loss)
Total other income (loss)		(3,413,000)		2,407,000
Income tax expense (benefit)		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		4,219,000		(1,801,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0
Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking		0		0
Brokerage		(1,932,000)		(69,000)
Management fees	[1]	(5,503,000)		(5,985,000)
Incentive income	[1]	(5,652,000)		(12,808,000)
Investment Income	[2]	(1,850,000)		(28,880,000)
Interest and dividends	[2]	3,653,000		3,083,000
Reimbursement from affiliates	[3]	3,901,000		3,727,000
Other revenue	[2]	1,772,000		599,000
Total revenues		(5,611,000)		(40,333,000)
Expenses
Non-interest expense	[2],[5]	2,791,000	[4]	1,506,000
Interest and dividends	[2]	3,037,000		1,757,000
Total expenses		5,828,000		3,263,000
Other income (loss)
Total other income (loss)	[2]	6,998,000		41,670,000
Income tax expense (benefit)	[4]	(3,320,000)		6,947,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		2,305,000		1,926,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		1,184,000		(6,947,000)
Consolidated Funds
Revenues
Total revenues		1,551,000		258,000
Expenses
Total expenses		1,816,000		358,000
Consolidated Funds | Reported Under Economic Income / (Loss)
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Consolidated Funds | Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Consolidated Funds | Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Total revenues		1,551,000		258,000
Expenses
Total expenses		1,816,000		358,000
Consolidated Funds | Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Total revenues		0		0
Expenses
Total expenses		$ 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] Reimbursement from affiliates is shown as a reduction of Economic Income expenses, but is included as a part of revenues under US GAAP

[4] Economic Income (Loss) excludes income taxes and acquisition related adjustments as management does not consider these items when evaluating the performance of the segment.

[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.