Segment Reporting (Details)		3 Months Ended				9 Months Ended
		Sep. 30, 2017 USD ($)		Sep. 30, 2016 USD ($)		Sep. 30, 2017 USD ($) segment customer		Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer						0
Number of Operating Segments | segment						2
Revenues
Investment banking		$ 57,383,000		$ 36,722,000		$ 158,082,000		$ 98,156,000
Brokerage		84,220,000		49,605,000		198,599,000		147,640,000
Management fees		8,223,000		10,272,000		25,587,000		31,951,000
Incentive income		1,945,000		1,284,000		6,217,000		2,823,000
Investment Income		0		0		0		0
Interest and dividends		14,318,000		3,906,000		27,324,000		11,664,000
Aircraft lease revenue		934,000		1,089,000		3,036,000		3,071,000
Reimbursement from affiliates		484,000		2,140,000		2,631,000		8,268,000
Reinsurance premiums		7,186,000		8,905,000		21,957,000		23,243,000
Other revenue		3,402,000		16,207,000		6,147,000		17,940,000
Total revenues		178,830,000		131,027,000		454,331,000		349,297,000
Expenses
Non-interest expense		177,504,000		148,178,000		478,033,000		363,630,000
Interest and dividends		15,132,000		7,612,000		37,273,000		21,866,000
Total expenses		194,348,000		158,259,000		524,729,000		391,924,000
Other income (loss)
Total other income (loss)		27,627,000		51,250,000		124,307,000		16,177,000
Income tax expense (benefit)		2,281,000		8,759,000		3,407,000		(6,553,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(5,162,000)		(18,478,000)		(35,412,000)		2,524,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		4,666,000		(3,219,000)		15,090,000		(17,373,000)
Consolidated Funds
Revenues
Total revenues		735,000		897,000		4,751,000		4,541,000
Expenses
Total expenses		1,712,000		2,469,000		9,423,000		6,428,000
Operating Segments
Revenues
Investment banking		57,383,000		36,722,000		158,082,000		98,156,000
Brokerage		89,968,000		51,524,000		209,274,000		153,417,000
Management fees		13,794,000		16,539,000		42,095,000		49,938,000
Incentive income		4,597,000		11,825,000		18,626,000		19,174,000
Investment Income		15,949,000		19,730,000		51,765,000		(591,000)
Interest and dividends		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenue		939,000		17,170,000		3,314,000		19,450,000
Total revenues		182,630,000		153,510,000		483,156,000		339,544,000
Expenses
Non-interest expense		169,065,000		135,848,000		440,316,000		335,889,000
Interest and dividends		4,785,000		4,284,000		13,453,000		12,933,000
Total expenses		173,850,000		140,132,000		453,769,000		348,822,000
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		(524,000)		(2,354,000)		(4,129,000)		(6,589,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		8,256,000		11,024,000		25,258,000		(15,867,000)
Operating Segments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		0		0		0		0
Operating Segments | Investment Management
Revenues
Investment banking		0		0		0		0
Brokerage		19,000		0		38,000		0
Management fees		12,983,000		15,746,000		39,730,000		47,604,000
Incentive income		4,597,000		11,825,000		18,626,000		19,174,000
Investment Income		9,927,000		14,798,000		34,412,000		(444,000)
Interest and dividends		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenue		387,000		17,145,000		2,067,000		19,366,000
Total revenues		27,913,000		59,514,000		94,873,000		85,700,000
Expenses
Non-interest expense		21,333,000		43,589,000		68,524,000		77,917,000
Interest and dividends		3,662,000		3,205,000		10,141,000		9,645,000
Total expenses		24,995,000		46,794,000		78,665,000		87,562,000
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		(524,000)		(2,354,000)		(4,129,000)		(6,589,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		2,394,000		10,366,000		12,079,000		(8,451,000)
Operating Segments | Investment Management | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		0		0		0		0
Operating Segments | Broker-Dealer
Revenues
Investment banking		57,383,000		36,722,000		158,082,000		98,156,000
Brokerage		89,949,000		51,524,000		209,236,000		153,417,000
Management fees		811,000		793,000		2,365,000		2,334,000
Incentive income		0		0		0		0
Investment Income		6,022,000		4,932,000		17,353,000		(147,000)
Interest and dividends		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenue		552,000		25,000		1,247,000		84,000
Total revenues		154,717,000		93,996,000		388,283,000		253,844,000
Expenses
Non-interest expense		147,732,000		92,259,000		371,792,000		257,972,000
Interest and dividends		1,123,000		1,079,000		3,312,000		3,288,000
Total expenses		148,855,000		93,338,000		375,104,000		261,260,000
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.		5,862,000		658,000		13,179,000		(7,416,000)
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		(640,000)		(445,000)		(1,773,000)		(1,239,000)
Incentive income		(402,000)		(562,000)		(3,669,000)		(569,000)
Investment Income		0		0		0		0
Interest and dividends		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reimbursement from affiliates		(88,000)		(76,000)		(241,000)		(224,000)
Reinsurance premiums		0		0		0		0
Other revenue		0		0		0		0
Total revenues		(395,000)		(186,000)		(932,000)		2,509,000
Expenses
Non-interest expense		0		(429,000)		(199,000)		(429,000)
Interest and dividends		0		0		0		0
Total expenses		1,712,000		2,040,000		9,224,000		5,999,000
Other income (loss)
Total other income (loss)		7,535,000		19,269,000		44,111,000		(1,915,000)
Income tax expense (benefit)		0		0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(5,428,000)		(17,043,000)		(33,955,000)		5,405,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		735,000		897,000		4,751,000		4,541,000
Expenses
Total expenses		1,712,000		2,469,000		9,423,000		6,428,000
Adjustments | Other Adjustments
Revenues
Investment banking		0		0		0		0
Brokerage	[1]	(5,748,000)		(1,919,000)		(10,675,000)		(5,777,000)
Management fees	[2]	(4,931,000)		(5,822,000)		(14,735,000)		(16,748,000)
Incentive income	[2]	(2,250,000)		(9,979,000)		(8,740,000)		(15,782,000)
Investment Income	[3],[4]	(15,949,000)		(19,730,000)		(51,765,000)		591,000
Interest and dividends	[4]	14,318,000		3,906,000		27,324,000		11,664,000
Aircraft lease revenue	[3]	934,000		1,089,000		3,036,000		3,071,000
Reimbursement from affiliates	[5]	572,000		2,216,000		2,872,000		8,492,000
Reinsurance premiums	[6]	7,186,000		8,905,000		21,957,000		23,243,000
Other revenue		2,463,000	[6]	(963,000)	[4]	2,833,000	[6]	(1,510,000)	[6]
Total revenues		(3,405,000)		(22,297,000)		(27,893,000)		7,244,000
Expenses
Non-interest expense	[7],[8]	8,439,000	[4]	12,759,000	[4]	37,916,000		28,170,000
Interest and dividends	[4]	10,347,000		3,328,000		23,820,000		8,933,000
Total expenses		18,786,000		16,087,000		61,736,000		37,103,000
Other income (loss)
Total other income (loss)	[4]	20,092,000	[9]	31,981,000		80,196,000	[9]	18,092,000
Income tax expense (benefit)	[7]	2,281,000		8,759,000		3,407,000		(6,553,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		790,000		919,000		2,672,000		3,708,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(3,590,000)		(14,243,000)		(10,168,000)		(1,506,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		$ 0		$ 0		$ 0		$ 0

[1] Economic Income (Loss) brokerage revenues included net securities borrowed and securities loaned activities.

[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, the healthcare royalty business and the activist business.

[3] Aircraft lease revenue is shown net of expenses in investment income for Economic Income (Loss).

[4] Economic Income (Loss) recognizes Company income from proprietary trading (including interest and dividends).

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

[6] Economic Income (Loss) recognizes underwriting income from the Company's insurance related activities, net of expenses, within other revenue.

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

[8] Economic Income (Loss) recognizes the Company's proportionate share of expenses, for certain real estate operating entities and the activist business, for which the investments are recorded under the equity method of accounting for investments.

[9] Economic Income (Loss) excludes the bargain purchase gain which resulted from the Convergex Group acquisition.