Segment Reporting (Details) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2015 customer segment		Mar. 31, 2014 customer		Dec. 31, 2013 customer
Segment Reporting Information [Line Items]
Number of Operating Segments	2
Revenues
Investment banking	$ 65,233,000		$ 49,562,000
Brokerage	35,454,000		32,830,000
Management fees	10,384,000		8,924,000
Incentive income	2,372,000		2,498,000
Investment Income	0		0
Interest and dividends	3,083,000		9,252,000
Reimbursement from affiliates	3,642,000		1,900,000
Other revenue	668,000		555,000
Total revenues	121,094,000		106,677,000
Expenses
Non-interest expense	132,669,000		98,654,000
Interest and dividends	5,779,000		7,072,000
Total expenses	138,806,000		106,028,000
Other income (loss)
Total other income (loss)	44,077,000		13,457,000
Income tax expense (benefit)	6,947,000		79,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(2,720,000)		(4,187,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	16,698,000		9,840,000
Number of major customers	0		0		0
Reported Under Economic Income / (Loss)
Revenues
Investment banking	65,233,000		49,562,000
Brokerage	35,523,000		34,349,000
Management fees	16,607,000		14,089,000
Incentive income	15,363,000		4,594,000
Investment Income	28,880,000		8,172,000
Interest and dividends	0		0
Reimbursement from affiliates	0		0
Other revenue	69,000		(145,000)
Total revenues	161,675,000		110,621,000
Expenses
Non-interest expense	131,163,000		97,351,000
Interest and dividends	4,022,000		643,000
Total expenses	135,185,000		97,994,000
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	(2,845,000)		(2,625,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	23,645,000		10,002,000
Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Investment banking	0		0
Brokerage	18,000		0
Management fees	16,607,000		14,089,000
Incentive income	15,363,000		4,594,000
Investment Income	21,830,000		8,328,000
Interest and dividends	0		0
Reimbursement from affiliates	0		0
Other revenue	21,000		(43,000)
Total revenues	53,839,000		26,968,000
Expenses
Non-interest expense	34,834,000		21,172,000
Interest and dividends	3,044,000		609,000
Total expenses	37,878,000		21,781,000
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	(2,845,000)		(2,625,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	13,116,000		2,562,000
Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Investment banking	65,233,000		49,562,000
Brokerage	35,505,000		34,349,000
Management fees	0		0
Incentive income	0		0
Investment Income	7,050,000		(156,000)
Interest and dividends	0		0
Reimbursement from affiliates	0		0
Other revenue	48,000		(102,000)
Total revenues	107,836,000		83,653,000
Expenses
Non-interest expense	96,329,000		76,179,000
Interest and dividends	978,000		34,000
Total expenses	97,307,000		76,213,000
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	0		0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	10,529,000		7,440,000
Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking	0		0
Brokerage	0		0
Management fees	(238,000)		(234,000)
Incentive income	(183,000)		0
Investment Income	0		0
Interest and dividends	0		0
Reimbursement from affiliates	(85,000)		(80,000)
Other revenue	0		0
Total revenues	(248,000)		842,000
Expenses
Non-interest expense	0		0
Interest and dividends	0		0
Total expenses	358,000		302,000
Other income (loss)
Total other income (loss)	2,407,000		207,000
Income tax expense (benefit)	0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,801,000)		(747,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	0		0
Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking	0		0
Brokerage	(69,000)	[1]	(1,519,000)	[1]
Management fees	(5,985,000)	[2]	(4,931,000)	[2]
Incentive income	(12,808,000)	[2]	(2,096,000)	[2]
Investment Income	(28,880,000)	[3]	(8,172,000)	[3]
Interest and dividends	3,083,000	[1],[3]	9,252,000	[1],[3]
Reimbursement from affiliates	3,727,000	[4]	1,980,000	[4]
Other revenue	599,000	[3]	700,000	[3]
Total revenues	(40,333,000)		(4,786,000)
Expenses
Non-interest expense	1,506,000	[3],[5]	1,303,000	[3],[5]
Interest and dividends	1,757,000	[1],[3]	6,429,000	[1],[3]
Total expenses	3,263,000		7,732,000
Other income (loss)
Total other income (loss)	41,670,000	[3]	13,250,000	[3]
Income tax expense (benefit)	6,947,000	[6]	79,000	[6]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	1,926,000		(815,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc. stockholders	(6,947,000)		(162,000)
Consolidated Funds
Revenues
Total revenues	258,000		1,156,000
Expenses
Total expenses	358,000		302,000
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	258,000		1,156,000
Expenses
Total expenses	358,000		302,000
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 stock borrow/loan activity 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

[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.