Segment Reporting (Details)		3 Months Ended								12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment customer	Dec. 31, 2014 USD ($) customer	Dec. 31, 2013 USD ($) customer
Segment Reporting Information [Line Items]
Number of Operating Segments | segment										2
Revenues
Investment banking										$ 222,781,000	$ 170,506,000	$ 105,333,000
Brokerage										157,722,000	140,132,000	114,593,000
Management fees										41,906,000	40,627,000	37,303,000
Incentive income										1,466,000	2,785,000	12,586,000
Investment Income										0	0	0
Interest and dividends										13,796,000	48,870,000	39,454,000
Reimbursement from affiliates										21,557,000	12,495,000	10,434,000
Other revenue										3,726,000	9,446,000	5,418,000
Total revenues		$ 110,611,000	$ 113,254,000	$ 119,608,000	$ 121,094,000	$ 121,096,000	$ 107,101,000	$ 92,902,000	$ 106,677,000	464,567,000	427,776,000	328,519,000
Expenses
Non-interest expense										475,844,000	443,471,000	331,579,000
Goodwill Impairment										0	2,334,000	0
Interest and dividends										26,220,000	42,752,000	27,299,000
Total expenses										504,374,000	490,191,000	360,917,000
Other income (loss)
Total other income (loss)										51,286,000	120,251,000	50,695,000
Income tax expense (benefit)		(52,708,000)	(5,081,000)	3,346,000	6,947,000	(125,210,000)	141,000	46,000	79,000	(47,496,000)	(124,944,000)	457,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(15,246,000)	(15,564,000)	(13,193,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		$ 30,661,000	$ (10,346,000)	$ 6,716,000	$ 16,698,000	$ 142,471,000	$ 6,523,000	$ 8,382,000	$ 9,840,000	$ 43,729,000	$ 167,216,000	$ 4,647,000
Number of major customers | customer										0	0	0
Reported Under Economic Income / (Loss)
Revenues
Investment banking										$ 222,781,000	$ 170,506,000	$ 105,333,000
Brokerage										160,436,000	146,247,000	121,065,000
Management fees										70,015,000	64,774,000	56,984,000
Incentive income										(1,544,000)	45,708,000	21,205,000
Investment Income										62,596,000	65,215,000	36,660,000
Interest and dividends										0	0	0
Reimbursement from affiliates										0	0	0
Other revenue										15,382,000	5,168,000	2,534,000
Total revenues										529,666,000	497,618,000	343,781,000
Expenses
Non-interest expense										469,754,000	435,862,000	323,895,000
Goodwill Impairment											0
Interest and dividends										16,584,000	9,798,000	350,000
Total expenses										486,338,000	445,660,000	324,245,000
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 and funds										(8,796,000)	(7,802,000)	(12,995,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										34,532,000	44,156,000	6,541,000
Alternative Investment
Expenses
Goodwill Impairment										0	0
Alternative Investment | Reported Under Economic Income / (Loss)
Revenues
Investment banking										0	0	0
Brokerage										0	55,000	0
Management fees										68,989,000	64,774,000	56,984,000
Incentive income										(1,544,000)	45,708,000	21,205,000
Investment Income										49,244,000	45,193,000	30,713,000
Interest and dividends										0	0	0
Reimbursement from affiliates										0	0	0
Other revenue										14,492,000	4,645,000	524,000
Total revenues										131,181,000	160,375,000	109,426,000
Expenses
Non-interest expense										107,291,000	115,601,000	86,054,000
Goodwill Impairment											0
Interest and dividends										11,839,000	7,804,000	231,000
Total expenses										119,130,000	123,405,000	86,285,000
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 and funds										(8,796,000)	(7,802,000)	(12,995,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										3,255,000	29,168,000	10,146,000
Broker-Dealer
Expenses
Goodwill Impairment										0	2,334,000
Broker-Dealer | Reported Under Economic Income / (Loss)
Revenues
Investment banking										222,781,000	170,506,000	105,333,000
Brokerage										160,436,000	146,192,000	121,065,000
Management fees										1,026,000	0	0
Incentive income										0	0	0
Investment Income										13,352,000	20,022,000	5,947,000
Interest and dividends										0	0	0
Reimbursement from affiliates										0	0	0
Other revenue										890,000	523,000	2,010,000
Total revenues										398,485,000	337,243,000	234,355,000
Expenses
Non-interest expense										362,463,000	320,261,000	237,841,000
Goodwill Impairment											0
Interest and dividends										4,745,000	1,994,000	119,000
Total expenses										367,208,000	322,255,000	237,960,000
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 and funds										0	0	0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										31,277,000	14,988,000	(3,605,000)
Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking										0	0	0
Brokerage										0	0	0
Management fees										(1,307,000)	(963,000)	(1,146,000)
Incentive income										(736,000)	(281,000)	0
Investment Income										0	0	0
Interest and dividends										0	0	0
Reimbursement from affiliates										(190,000)	(342,000)	(99,000)
Other revenue										0	0	0
Total revenues										(620,000)	1,329,000	2,153,000
Expenses
Non-interest expense										0	0	0
Goodwill Impairment											0
Interest and dividends										0	0	0
Total expenses										2,310,000	1,634,000	2,039,000
Other income (loss)
Total other income (loss)										8,781,000	5,775,000	2,618,000
Income tax expense (benefit)										0	0	0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(5,851,000)	(5,470,000)	(2,732,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										0	0	0
Significant Reconciling Items | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Investment banking										0	0	0
Brokerage	[1]									(2,714,000)	(6,115,000)	(6,472,000)
Management fees	[2]									(26,802,000)	(23,184,000)	(18,535,000)
Incentive income	[2]									3,746,000	(42,642,000)	(8,619,000)
Investment Income	[3]									(62,596,000)	(65,215,000)	(36,660,000)
Interest and dividends	[1],[3]									13,796,000	48,870,000	39,454,000
Reimbursement from affiliates	[4]									21,747,000	12,837,000	10,533,000
Other revenue	[3]									(11,656,000)	4,278,000	2,884,000
Total revenues										(64,479,000)	(71,171,000)	(17,415,000)
Expenses
Non-interest expense	[3],[5]									6,090,000	7,609,000	7,684,000
Goodwill Impairment	[6]										2,334,000
Interest and dividends	[1],[3]									9,636,000	32,954,000	26,949,000
Total expenses										15,726,000	42,897,000	34,633,000
Other income (loss)
Total other income (loss)	[3]									42,505,000	114,476,000	48,077,000
Income tax expense (benefit)	[7]									(47,496,000)	(124,944,000)	457,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(599,000)	(2,292,000)	2,534,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										9,197,000	123,060,000	(1,894,000)
Consolidated Funds
Revenues
Total revenues										1,613,000	2,915,000	3,398,000
Expenses
Total expenses										2,310,000	1,634,000	2,039,000
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 | Funds Consolidation | Reconciliation from Economic Income / (Loss) to U.S. GAAP
Revenues
Total revenues										1,613,000	2,915,000	3,398,000
Expenses
Total expenses										2,310,000	1,634,000	2,039,000
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

[1] Economic Income (Loss) recognizes stock borrow/loan activity (prior to January 2015) 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 of related expenses.

[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 goodwill impairment and other reorganization expenses.

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