Segment Reporting (Details)		3 Months Ended				9 Months Ended				12 Months Ended
		Sep. 30, 2019 USD ($)		Sep. 30, 2018 USD ($)		Sep. 30, 2019 USD ($) segment customer		Sep. 30, 2018 USD ($)		Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment						2
Entity-wide revenue, major customer, number | customer						0
Revenues
Investment banking		$ 77,292,000		$ 81,923,000		$ 272,103,000		$ 264,737,000
Brokerage		93,995,000		92,035,000		302,840,000		301,053,000
Management fees		7,300,000		7,546,000		21,480,000		22,336,000
Incentive income		701,000		33,000		724,000		97,000
Investment Income		0		0		0		0
Interest and dividends		60,707,000		26,448,000		129,846,000		77,511,000
Reimbursement from affiliates		238,000		353,000		780,000		1,066,000
Aircraft lease revenue		0		375,000		0		1,509,000
Reinsurance premiums		8,146,000		8,378,000		29,068,000		26,251,000
Other revenues		1,237,000		1,634,000		3,228,000		3,846,000
Total revenues		252,047,000		221,028,000		768,308,000		706,985,000
Interest expense		56,477,000		26,000,000		125,089,000		74,846,000
Total net revenues		195,570,000		195,028,000		643,219,000		632,139,000
Expenses
Non-interest expense		217,448,000		210,642,000		679,655,000		656,299,000
Goodwill Impairment		0		0		4,100,000		0		$ 0
Total expenses		219,964,000		212,329,000		689,984,000		663,566,000
Other income (loss)
Total other income (loss)		32,342,000		41,982,000		82,976,000		122,964,000
Income tax expense (benefit)		1,365,000		5,083,000		9,615,000		15,999,000
(Net Income (Loss) Attributable to Noncontrolling Interest		(2,770,000)		(4,110,000)		(7,188,000)		(37,798,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		3,813,000		15,488,000		19,408,000		37,740,000
Preferred stock dividends		1,698,000		1,698,000		5,094,000		5,094,000
Net income (loss) attributable to Cowen Inc. common stockholders		2,115,000		13,790,000		14,314,000		32,646,000
Consolidated Funds
Revenues
Total revenues		2,431,000		2,303,000		8,239,000		8,579,000
Expenses
Total expenses		2,516,000		1,687,000		6,229,000		7,267,000
Other income (loss)
(Net Income (Loss) Attributable to Noncontrolling Interest		(4,896,000)		(2,804,000)		(8,774,000)		(34,993,000)
Operating Company
Revenues
Management fees		7,052,000		6,595,000		20,492,000		19,556,000
Incentive income		701,000		33,000		710,000		97,000
Asset Company
Revenues
Management fees		248,000		951,000		988,000		2,780,000
Incentive income		0		0		14,000		0
Operating Segments
Revenues
Investment banking		70,022,000		77,510,000		257,234,000		251,476,000
Brokerage		105,860,000		101,672,000		331,689,000		328,929,000
Management fees		10,907,000		12,386,000		31,804,000		37,998,000
Incentive income		14,377,000		6,910,000		35,353,000		21,470,000
Investment Income		16,053,000		27,672,000		33,522,000		61,927,000
Interest and dividends		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Aircraft lease revenue				0				0
Reinsurance premiums		0		0		0		0
Other revenues		(126,000)		83,000		5,351,000		250,000
Total revenues		217,093,000		226,233,000		694,953,000		702,050,000
Interest expense		7,147,000		6,419,000		20,417,000		16,500,000
Total net revenues		209,946,000		219,814,000		674,536,000		685,550,000
Expenses
Non-interest expense		202,040,000		196,188,000		630,192,000		613,561,000
Goodwill Impairment						0
Total expenses		202,040,000		196,188,000		630,192,000		613,561,000
Other income (loss)
Total other income (loss)		0		0		0		0
Income tax expense (benefit)		0		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		(661,000)		(942,000)		(2,944,000)		(3,505,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		7,245,000		22,684,000		41,400,000		68,484,000
Preferred stock dividends		1,698,000		1,698,000		5,094,000		5,094,000
Net income (loss) attributable to Cowen Inc. common stockholders		5,547,000		20,986,000		36,306,000		63,390,000
Other Depreciation and Amortization		5,082,000		2,908,000		14,987,000		8,854,000
Economic Operating Income		10,629,000		23,894,000		51,293,000		72,244,000
Operating Segments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		0		0		0		0
Operating Segments | Operating Company
Revenues
Investment banking		70,022,000		77,510,000		257,234,000		251,476,000
Brokerage		105,860,000		101,672,000		331,689,000		328,929,000
Management fees		10,321,000		10,840,000		30,016,000		33,057,000
Incentive income		15,384,000		1,859,000		34,654,000		17,219,000
Investment Income		15,231,000		25,779,000		28,233,000		62,488,000
Interest and dividends		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Aircraft lease revenue				0				0
Reinsurance premiums		0		0		0		0
Other revenues		(132,000)		4,000		5,295,000		(143,000)
Total revenues		216,686,000		217,664,000		687,121,000		693,026,000
Interest expense		5,758,000		4,878,000		16,371,000		12,539,000
Total net revenues		210,928,000		212,786,000		670,750,000		680,487,000
Expenses
Non-interest expense		200,637,000		189,516,000		623,167,000		601,480,000
Goodwill Impairment						0
Total expenses		200,637,000		189,516,000		623,167,000		601,480,000
Other income (loss)
Total other income (loss)		0		0		0		0
Income tax expense (benefit)		0		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		(661,000)		(898,000)		(2,944,000)		(2,805,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		9,630,000		22,372,000		44,639,000		76,202,000
Preferred stock dividends		1,341,000		1,290,000		4,058,000		3,871,000
Net income (loss) attributable to Cowen Inc. common stockholders		8,289,000		21,082,000		40,581,000		72,331,000
Other Depreciation and Amortization		5,073,000		2,865,000		14,957,000		8,707,000
Economic Operating Income		13,362,000		23,947,000		55,538,000		81,038,000
Operating Segments | Operating Company | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		0		0		0		0
Operating Segments | Asset Company
Revenues
Investment banking		0		0		0		0
Brokerage		0		0		0		0
Management fees		586,000		1,546,000		1,788,000		4,941,000
Incentive income		(1,007,000)		5,051,000		699,000		4,251,000
Investment Income		822,000		1,893,000		5,289,000		(561,000)
Interest and dividends		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Aircraft lease revenue				0				0
Reinsurance premiums		0		0		0		0
Other revenues		6,000		79,000		56,000		393,000
Total revenues		407,000		8,569,000		7,832,000		9,024,000
Interest expense		1,389,000		1,541,000		4,046,000		3,961,000
Total net revenues		(982,000)		7,028,000		3,786,000		5,063,000
Expenses
Non-interest expense		1,403,000		6,672,000		7,025,000		12,081,000
Goodwill Impairment						0
Total expenses		1,403,000		6,672,000		7,025,000		12,081,000
Other income (loss)
Total other income (loss)		0		0		0		0
Income tax expense (benefit)		0		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		0		(44,000)		0		(700,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(2,385,000)		312,000		(3,239,000)		(7,718,000)
Preferred stock dividends		357,000		408,000		1,036,000		1,223,000
Net income (loss) attributable to Cowen Inc. common stockholders		(2,742,000)		(96,000)		(4,275,000)		(8,941,000)
Other Depreciation and Amortization		9,000		43,000		30,000		147,000
Economic Operating Income		(2,733,000)		(53,000)		(4,245,000)		(8,794,000)
Operating Segments | Asset Company | 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		(574,000)		(381,000)		(1,640,000)		(2,197,000)
Incentive income		(13,000)		(20,000)		(557,000)		(37,000)
Investment Income		0		0		0		0
Interest and dividends		0		0		0		0
Reimbursement from affiliates		(27,000)		(65,000)		(94,000)		(198,000)
Aircraft lease revenue				0				0
Reinsurance premiums		0		0		0		0
Other revenues		(26,000)		0		(14,000)		0
Total revenues		1,791,000		1,837,000		5,934,000		6,147,000
Interest expense		0		0		0		0
Total net revenues		1,791,000		1,837,000		5,934,000		6,147,000
Expenses
Non-interest expense		0		0		0		0
Goodwill Impairment						0
Total expenses		2,516,000		1,687,000		6,229,000		7,267,000
Other income (loss)
Total other income (loss)		5,621,000		2,654,000		9,069,000		36,113,000
Income tax expense (benefit)		0		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		(4,896,000)		(2,804,000)		(8,774,000)		(34,993,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0		0		0
Preferred stock dividends		0		0		0		0
Net income (loss) attributable to Cowen Inc. common stockholders		0		0		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		2,431,000		2,303,000		8,239,000		8,579,000
Expenses
Total expenses		2,516,000		1,687,000		6,229,000		7,267,000
Adjustments | Other Adjustments
Revenues
Investment banking	[1]	7,270,000		4,413,000		14,869,000		13,261,000
Brokerage	[2]	(11,865,000)		(9,637,000)		(28,849,000)		(27,876,000)
Management fees	[3]	(3,033,000)		(4,459,000)		(8,684,000)		(13,465,000)
Incentive income	[3]	(13,663,000)		(6,857,000)	[1]	(34,072,000)		(21,336,000)
Investment Income	[4]	(16,053,000)		(27,672,000)		(33,522,000)		(61,927,000)	[5]
Interest and dividends	[4]	60,707,000	[2]	26,448,000		129,846,000	[2]	77,511,000	[2]
Reimbursement from affiliates	[6]	265,000		418,000		874,000		1,264,000
Aircraft lease revenue	[5]			375,000				1,509,000
Reinsurance premiums	[7]	8,146,000		8,378,000		29,068,000		26,251,000
Other revenues	[7]	1,389,000		1,551,000		(2,109,000)		3,596,000
Total revenues		33,163,000		(7,042,000)		67,421,000		(1,212,000)
Interest expense	[2],[4]	49,330,000		19,581,000		104,672,000		58,346,000
Total net revenues		(16,167,000)		(26,623,000)		(37,251,000)		(59,558,000)
Expenses
Non-interest expense	[1],[6],[8],[9]	15,408,000		14,454,000		49,463,000		42,738,000
Goodwill Impairment						4,100,000
Total expenses		15,408,000		14,454,000		53,563,000		42,738,000
Other income (loss)
Total other income (loss)	[3],[4],[9]	26,721,000		39,328,000	[10]	73,907,000		86,851,000	[10]
Income tax expense (benefit)		1,365,000	[8]	5,083,000	[10]	9,615,000	[8]	15,999,000	[8]
(Net Income (Loss) Attributable to Noncontrolling Interest		2,787,000		(364,000)		4,530,000		700,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(3,432,000)		(7,196,000)		(21,992,000)		(30,744,000)
Preferred stock dividends		0		0		0		0
Net income (loss) attributable to Cowen Inc. common stockholders		(3,432,000)		(7,196,000)		(21,992,000)		(30,744,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		$ 0		$ 0		$ 0		$ 0

[1] Economic Income (Loss) presents underwriting expenses net of investment banking revenues, expenses reimbursed from clients within their respective expense category. Economic Income (Loss) also records retainer fees, relating to investment banking activities, collectible during the period that would otherwise be deferred until closing for US GAAP reporting.

[2] Economic Income (Loss) brokerage revenues included net securities borrowed and securities loaned activities which are shown gross in interest income and interest expense for US GAAP.

[3] Economic Income (Loss) recognizes revenues (i) net of distribution fees paid to agents, (ii) records income from uncrystallized incentive fees and (iii) the Company's proportionate share of management and incentive fees of certain real estate operating entities, the healthcare royalty business and the activist business.

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

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

[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 underwriting income from the Company's insurance related activities, net of expenses, within other revenue.

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

[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.

[10] Economic Income (Loss) excludes gain/(loss) on debt extinguishment.