Segment Reporting (Details)		3 Months Ended
		Mar. 31, 2018 USD ($) segment customer		Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer		0
Number of Operating Segments | segment		2
Revenues
Investment banking		$ 97,988,000		$ 36,553,000
Brokerage		105,733,000		50,534,000
Management fees		7,417,000		8,708,000
Incentive income		16,000		546,000
Investment Income		0		0
Interest and dividends		25,954,000		5,089,000
Aircraft lease revenue		715,000		1,059,000
Reimbursement from affiliates		377,000		1,652,000
Reinsurance premiums		8,647,000		7,089,000
Other revenues		1,336,000		1,400,000
Total revenues		251,384,000		114,971,000
Expenses
Non-interest expense		223,315,000		125,480,000
Interest and dividends		24,540,000		9,930,000
Total expenses		250,286,000		140,373,000
Other income (loss)
Total other income (loss)		33,834,000		39,402,000
Income tax expense (benefit)		6,923,000		1,911,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(11,156,000)		(9,105,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		16,853,000		2,984,000
Consolidated Funds
Revenues
Total revenues		3,201,000		2,341,000
Expenses
Total expenses		2,431,000		4,963,000
Operating Segments
Revenues
Investment banking		93,924,000		36,553,000
Brokerage		114,071,000		52,313,000
Management fees		13,126,000		13,946,000
Incentive income		5,197,000		3,060,000
Investment Income		14,301,000		21,626,000
Interest and dividends		0		0
Aircraft lease revenue		0		0
Reimbursement from affiliates		0		0
Reinsurance premiums		0		0
Other revenues		888,000		1,104,000
Total revenues		241,507,000		128,602,000
Expenses
Non-interest expense		209,333,000		117,158,000
Interest and dividends		5,958,000		4,341,000
Total expenses		215,291,000		121,499,000
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		(2,158,000)		(1,645,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		24,058,000		5,458,000
Operating Segments | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Operating Segments | Investment Management
Revenues
Investment banking		0		0
Brokerage		0		0
Management fees		12,355,000		13,145,000
Incentive income		5,197,000		3,060,000
Investment Income		11,896,000		15,149,000
Interest and dividends		0		0
Aircraft lease revenue		0		0
Reimbursement from affiliates		0		0
Reinsurance premiums		0		0
Other revenues		361,000		819,000
Total revenues		29,809,000		32,173,000
Expenses
Non-interest expense		22,881,000		23,191,000
Interest and dividends		3,854,000		3,241,000
Total expenses		26,735,000		26,432,000
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		(2,158,000)		(1,645,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		916,000		4,096,000
Operating Segments | Investment Management | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Operating Segments | Broker-Dealer
Revenues
Investment banking		93,924,000		36,553,000
Brokerage		114,071,000		52,313,000
Management fees		771,000		801,000
Incentive income		0		0
Investment Income		2,405,000		6,477,000
Interest and dividends		0		0
Aircraft lease revenue		0		0
Reimbursement from affiliates		0		0
Reinsurance premiums		0		0
Other revenues		527,000		285,000
Total revenues		211,698,000		96,429,000
Expenses
Non-interest expense		186,452,000		93,967,000
Interest and dividends		2,104,000		1,100,000
Total expenses		188,556,000		95,067,000
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.		23,142,000		1,362,000
Operating Segments | Broker-Dealer | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Adjustments | Funds Consolidation
Revenues
Investment banking		0		0
Brokerage		0		0
Management fees		(1,203,000)		(527,000)
Incentive income		(9,000)		(489,000)
Investment Income		0		0
Interest and dividends		0		0
Aircraft lease revenue		0		0
Reimbursement from affiliates		(68,000)		(78,000)
Reinsurance premiums		0		0
Other revenues		0		0
Total revenues		1,905,000		1,247,000
Expenses
Non-interest expense		0		(429,000)
Interest and dividends		0		0
Total expenses		2,431,000		4,534,000
Other income (loss)
Total other income (loss)		10,063,000		11,674,000
Income tax expense (benefit)		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(9,537,000)		(8,387,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		3,185,000		2,341,000
Expenses
Total expenses		2,431,000		4,963,000
Adjustments | Other Adjustments
Revenues
Investment banking		4,064,000		0
Brokerage	[1]	(8,338,000)		(1,779,000)
Management fees	[2]	(4,506,000)		(4,711,000)
Incentive income	[2]	(5,172,000)		(2,025,000)
Investment Income	[3],[4]	(14,301,000)		(21,626,000)
Interest and dividends	[4]	25,954,000		5,089,000
Aircraft lease revenue	[3]	715,000		1,059,000
Reimbursement from affiliates	[5]	445,000		1,730,000
Reinsurance premiums	[6]	8,647,000		7,089,000
Other revenues	[6]	449,000		296,000
Total revenues		7,957,000		(14,878,000)
Expenses
Non-interest expense	[7],[9]	13,966,000	[8]	8,751,000
Interest and dividends	[4]	18,582,000		5,589,000
Total expenses		32,548,000		14,340,000
Other income (loss)
Total other income (loss)	[4]	23,770,000		27,728,000
Income tax expense (benefit)	[7]	6,923,000		1,911,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		539,000		927,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(7,205,000)		(2,474,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		$ 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) presents underwriting expenses net of investment banking revenues and records income from uncrystallized incentive fees.

[9] 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.