Segment Reporting (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segment customer		Dec. 31, 2013 customer		Dec. 31, 2012 customer
Segment Reporting Information [Line Items]
Number of Operating Segments									2
Revenues
Investment banking									$ 170,506,000		$ 105,333,000		$ 71,762,000
Brokerage									140,132,000		114,593,000		91,167,000
Management fees									40,627,000		37,303,000		38,116,000
Incentive income									2,785,000		12,586,000		5,411,000
Investment Income									0		0		0
Interest and dividends									48,870,000		39,454,000		24,608,000
Reimbursement from affiliates									12,495,000		10,434,000		6,274,000
Other revenues									9,446,000		5,418,000		3,668,000
Total revenues	121,096,000	107,101,000	92,902,000	106,677,000	96,345,000	81,912,000	81,933,000	68,329,000	427,776,000		328,519,000		241,515,000
Expenses
Non-interest expense									443,471,000		331,579,000		313,087,000
Goodwill impairment									2,334,000		0		0
Interest and dividends									42,752,000		27,299,000		12,137,000
Total expenses									490,191,000		360,917,000		326,900,000
Other income (loss)
Total other income (loss)									120,251,000		50,695,000		61,876,000
Income tax expense (benefit)	(125,210,000)	141,000	46,000	79,000	169,000	(46,000)	158,000	176,000	(124,944,000)		457,000		448,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(15,564,000)		(13,193,000)		72,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	142,471,000	6,523,000	8,382,000	9,840,000	2,519,000	3,611,000	1,093,000	(2,576,000)	167,216,000		4,647,000		(23,885,000)
Number of major customers									0		0		0
Reported Under Economic Income / (Loss)
Revenues
Investment banking									170,506,000		105,333,000		71,762,000
Brokerage									146,247,000		121,065,000		93,903,000
Management fees									64,774,000		56,984,000		56,381,000
Incentive income									45,708,000		21,205,000		14,518,000
Investment Income									65,215,000		36,660,000		50,116,000
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									5,168,000		2,534,000		1,248,000
Total revenues									497,618,000		343,781,000		287,928,000
Expenses
Non-interest expense									435,862,000		323,895,000		302,678,000
Goodwill impairment									0
Interest and dividends									9,798,000		350,000		339,000
Total expenses									445,660,000		324,245,000		303,017,000
Other income (loss)
Total other income (loss)									0		0		0
Income tax expense (benefit)									0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(7,802,000)		(12,995,000)		(2,480,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders									44,156,000		6,541,000		(17,569,000)
Alternative Investment
Expenses
Goodwill impairment									0		0
Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Investment banking									0		0		0
Brokerage									55,000		0		0
Management fees									64,774,000		56,984,000		56,381,000
Incentive income									45,708,000		21,205,000		14,518,000
Investment Income									45,193,000		30,713,000		40,374,000
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									4,645,000		524,000		844,000
Total revenues									160,375,000		109,426,000		112,117,000
Expenses
Non-interest expense									115,601,000		86,054,000		90,949,000
Goodwill impairment									0
Interest and dividends									7,804,000		231,000		151,000
Total expenses									123,405,000		86,285,000		91,100,000
Other income (loss)
Total other income (loss)									0		0		0
Income tax expense (benefit)									0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(7,802,000)		(12,995,000)		(2,480,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders									29,168,000		10,146,000		18,537,000
Broker-Dealer
Expenses
Goodwill impairment									2,334,000		0
Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Investment banking									170,506,000		105,333,000		71,762,000
Brokerage									146,192,000		121,065,000		93,903,000
Management fees									0		0		0
Incentive income									0		0		0
Investment Income									20,022,000		5,947,000	[1]	9,742,000	[2]
Interest and dividends									0		0		0
Reimbursement from affiliates									0		0		0
Other revenue									523,000		2,010,000		404,000
Total revenues									337,243,000		234,355,000		175,811,000
Expenses
Non-interest expense									320,261,000		237,841,000	[1]	211,729,000	[2]
Goodwill impairment									0
Interest and dividends									1,994,000		119,000		188,000
Total expenses									322,255,000		237,960,000		211,917,000
Other income (loss)
Total other income (loss)									0		0		0
Income tax expense (benefit)									0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									0		0		0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders									14,988,000		(3,605,000)		(36,106,000)
Investment Income from Entity's Capital											7,600,000		10,200,000
Compensation Expense Related to Investment of Entity's Capital											2,500,000		3,400,000
Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking									0		0		0
Brokerage									(6,115,000)	[3]	(6,472,000)	[3]	(2,736,000)	[3]
Management fees									(23,184,000)	[4]	(18,535,000)	[4]	(16,791,000)	[4]
Incentive income									(42,642,000)	[4]	(8,619,000)	[4]	(9,107,000)	[4]
Investment Income									(65,215,000)	[5]	(36,660,000)	[5]	(50,116,000)	[5]
Interest and dividends									48,870,000	[3],[5]	39,454,000	[3],[5]	24,608,000	[3],[5]
Reimbursement from affiliates									12,837,000	[6]	10,533,000	[6]	6,562,000	[6]
Other revenue									4,278,000	[5]	2,884,000	[5]	2,420,000	[5]
Total revenues									(71,171,000)		(17,415,000)		(45,160,000)
Expenses
Non-interest expense									7,609,000	[5],[7],[8]	7,684,000	[5],[7]	10,409,000	[5],[7]
Goodwill impairment									2,334,000	[8]
Interest and dividends									32,954,000	[3],[5]	26,949,000	[3],[5]	11,798,000	[3],[5]
Total expenses									42,897,000		34,633,000		22,207,000
Other income (loss)
Total other income (loss)									114,476,000	[5]	48,077,000	[5]	59,320,000	[5]
Income tax expense (benefit)									(124,944,000)	[9]	457,000	[9]	448,000	[9]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(2,292,000)		2,534,000		2,179,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders									123,060,000		(1,894,000)		(6,316,000)
Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking									0		0		0
Brokerage									0		0		0
Management fees									(963,000)		(1,146,000)		(1,474,000)
Incentive income									(281,000)		0		0
Investment Income									0		0		0
Interest and dividends									0		0		0
Reimbursement from affiliates									(342,000)		(99,000)		(288,000)
Other revenue									0		0		0
Total revenues									1,329,000		2,153,000		(1,253,000)
Expenses
Non-interest expense									0		0		0
Goodwill impairment									0
Interest and dividends									0		0		0
Total expenses									1,634,000		2,039,000		1,676,000
Other income (loss)
Total other income (loss)									5,775,000		2,618,000		2,556,000
Income tax expense (benefit)									0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries									(5,470,000)		(2,732,000)		373,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders									0		0		0
Consolidated Funds
Revenues
Total revenues									2,915,000		3,398,000		509,000
Expenses
Total expenses									1,634,000		2,039,000		1,676,000
Consolidated Funds | Reported Under Economic Income / (Loss)
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Consolidated Funds | Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Consolidated Funds | Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Consolidated Funds | Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Total revenues									0		0		0
Expenses
Total expenses									0		0		0
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Total revenues									2,915,000		3,398,000		509,000
Expenses
Total expenses									$ 1,634,000		$ 2,039,000		$ 1,676,000

[1] For the year ended December 31, 2013, the Company has reflected $7.6 million of investment income and related compensation expense of $2.5 million within the broker-dealer segment in proportion to that segment's capital.

[2] For the year ended December 31, 2012, the Company has reflected $10.2 million of investment income and related compensation expense of $3.4 million within the broker-dealer segment in proportion to that segment's capital.

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

[4] 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.

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

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

[7] 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.

[8] Economic Income (Loss) excludes goodwill impairment and other reorganization expenses.

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