Segment Reporting (Details)		12 Months Ended
		Dec. 31, 2019 USD ($) segment customer		Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment		2
Entity-wide revenue, major customer, number | customer		0
Revenues
Investment banking		$ 375,025,000		$ 357,222,000		$ 223,614,000
Brokerage		402,747,000		413,582,000		293,610,000
Management fees		32,608,000		29,658,000		33,245,000
Incentive income		1,547,000		3,117,000		5,383,000
Investment Income		0		0		0
Interest and dividends		174,913,000		108,009,000		49,440,000
Reimbursement from affiliates		1,026,000		1,038,000		2,860,000
Aircraft lease revenue		0		1,852,000		3,751,000
Reinsurance premiums		46,335,000		38,096,000		30,996,000
Other revenues		5,433,000		4,504,000		8,561,000
Total revenues		1,049,443,000		966,916,000		658,781,000
Interest expense		168,628,000		104,116,000		60,949,000
Total net revenues		880,815,000		862,800,000		597,832,000
Expenses
Non-interest expense		935,801,000		882,329,000		684,123,000
Goodwill Impairment		4,100,000		0		0
Total expenses		948,864,000		890,944,000		696,649,000
Other income (loss)
Total other income (loss)		138,772,000		123,742,000		105,779,000
Income tax expense (benefit)		14,853,000		15,719,000		44,053,000
(Net Income (Loss) Attributable to Noncontrolling Interest		(31,239,000)		(37,060,000)		(23,791,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		24,631,000		42,819,000		(60,882,000)
Preferred stock dividends		6,792,000		6,792,000		6,792,000
Net income (loss) attributable to Cowen Inc. common stockholders		17,839,000		36,027,000		(67,674,000)
Consolidated Funds
Revenues
Total revenues		9,809,000		9,838,000		7,321,000
Expenses
Total expenses		8,963,000		8,615,000		12,526,000
Other income (loss)
(Net Income (Loss) Attributable to Noncontrolling Interest		(34,503,000)		(33,883,000)		(21,438,000)
Operating Company
Revenues
Management fees		31,361,000		26,080,000
Incentive income		1,532,000		3,117,000
Asset Company
Revenues
Management fees		1,248,000		3,578,000
Incentive income		15,000		0
Operating Segments
Revenues
Investment banking		352,192,000		329,061,000		223,614,000
Brokerage		440,413,000		452,299,000		312,780,000
Management fees		45,674,000		49,175,000		55,387,000
Incentive income		46,193,000		23,747,000		26,028,000
Investment Income		54,455,000		56,346,000		45,142,000
Interest and dividends		0		0		0
Reimbursement from affiliates		0		0		0
Aircraft lease revenue				0		0
Reinsurance premiums		0		0		0
Other revenues		5,843,000		(1,168,000)		3,231,000
Total revenues		944,770,000		909,460,000		666,182,000
Interest expense		28,025,000		23,013,000		18,888,000
Total net revenues		916,745,000		886,447,000		647,294,000
Expenses
Non-interest expense		856,515,000		805,359,000		625,387,000
Goodwill Impairment		0
Total expenses		856,515,000		805,359,000		625,387,000
Other income (loss)
Total other income (loss)		0		0		0
Income tax expense (benefit)		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		(4,796,000)		(4,954,000)		(6,072,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		55,434,000		76,134,000		15,835,000
Preferred stock dividends		6,792,000		6,792,000		6,792,000
Net income (loss) attributable to Cowen Inc. common stockholders		48,642,000		69,342,000		9,043,000
Other Depreciation and Amortization		20,439,000		11,583,000		11,558,000
Economic Operating Income		69,081,000		80,925,000		20,601,000
Operating Segments | Consolidated Funds
Revenues
Other revenues						0
Total revenues		0		0
Expenses
Total expenses		0		0		0
Operating Segments | Operating Company
Revenues
Investment banking		352,192,000		329,061,000		223,614,000
Brokerage		440,413,000		452,299,000		312,780,000
Management fees		43,698,000		43,466,000		45,007,000
Incentive income		45,041,000		16,851,000		17,872,000
Investment Income		51,344,000		53,593,000		7,204,000
Interest and dividends		0		0		0
Reimbursement from affiliates		0		0		0
Aircraft lease revenue				0		0
Reinsurance premiums		0		0		0
Other revenues		5,785,000		(1,619,000)		3,307,000
Total revenues		938,473,000		893,651,000		609,784,000
Interest expense		22,576,000		17,489,000		13,599,000
Total net revenues		915,897,000		876,162,000		596,185,000
Expenses
Non-interest expense		847,485,000		788,272,000		584,816,000
Goodwill Impairment		0
Total expenses		847,485,000		788,272,000		584,816,000
Other income (loss)
Total other income (loss)		0		0		0
Income tax expense (benefit)		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		(4,796,000)		(4,212,000)		(5,178,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		63,616,000		83,678,000		6,191,000
Preferred stock dividends		5,434,000		5,162,000		4,890,000
Net income (loss) attributable to Cowen Inc. common stockholders		58,182,000		78,516,000		1,301,000
Other Depreciation and Amortization		20,403,000		11,402,000		11,211,000
Economic Operating Income		78,585,000		89,918,000		12,512,000
Operating Segments | Operating Company | Consolidated Funds
Revenues
Other revenues						0
Total revenues		0		0
Expenses
Total expenses		0		0		0
Operating Segments | Asset Company
Revenues
Investment banking		0		0		0
Brokerage		0		0		0
Management fees		1,976,000		5,709,000		10,380,000
Incentive income		1,152,000		6,896,000		8,156,000
Investment Income		3,111,000		2,753,000		37,938,000
Interest and dividends		0		0		0
Reimbursement from affiliates		0		0		0
Aircraft lease revenue				0		0
Reinsurance premiums		0		0		0
Other revenues		58,000		451,000		(76,000)
Total revenues		6,297,000		15,809,000		56,398,000
Interest expense		5,449,000		5,524,000		5,289,000
Total net revenues		848,000		10,285,000		51,109,000
Expenses
Non-interest expense		9,030,000		17,087,000		40,571,000
Goodwill Impairment		0
Total expenses		9,030,000		17,087,000		40,571,000
Other income (loss)
Total other income (loss)		0		0		0
Income tax expense (benefit)		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		0		(742,000)		(894,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(8,182,000)		(7,544,000)		9,644,000
Preferred stock dividends		1,358,000		1,630,000		1,902,000
Net income (loss) attributable to Cowen Inc. common stockholders		(9,540,000)		(9,174,000)		7,742,000
Other Depreciation and Amortization		36,000		181,000		347,000
Economic Operating Income		(9,504,000)		(8,993,000)		8,089,000
Operating Segments | Asset Company | Consolidated Funds
Revenues
Other revenues						0
Total revenues		0		0
Expenses
Total expenses		0		0		0
Adjustments | Funds Consolidation
Revenues
Investment banking		0		0		0
Brokerage		0		0		0
Management fees		(2,270,000)		(2,513,000)		(2,593,000)
Incentive income		(600,000)		(52,000)		(1,739,000)
Investment Income		0		0		0
Interest and dividends		0		0		0
Reimbursement from affiliates		(122,000)		(269,000)		(297,000)
Aircraft lease revenue				0		0
Reinsurance premiums		0		0		0
Other revenues		(14,000)		0		0
Total revenues		6,803,000		7,004,000		2,692,000
Interest expense		0		0		0
Total net revenues		6,803,000		7,004,000		2,692,000
Expenses
Non-interest expense		0		0		(199,000)
Goodwill Impairment		0
Total expenses		8,963,000		8,615,000		12,327,000
Other income (loss)
Total other income (loss)		36,663,000		35,494,000		31,073,000
Income tax expense (benefit)		0		0		0
(Net Income (Loss) Attributable to Noncontrolling Interest		(34,503,000)		(33,883,000)		(21,438,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0		0
Preferred stock dividends		0		0
Net income (loss) attributable to Cowen Inc. common stockholders		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		9,809,000		9,838,000		7,321,000
Expenses
Total expenses		8,963,000		8,615,000		12,526,000
Adjustments | Other Adjustments
Revenues
Investment banking		22,833,000	[1]	28,161,000	[1]	0
Brokerage	[2]	(37,666,000)		(38,717,000)		(19,170,000)
Management fees	[3]	(10,796,000)		(17,004,000)		(19,549,000)
Incentive income	[3]	(44,046,000)		(20,578,000)		(18,906,000)
Investment Income	[4]	(54,455,000)		(56,346,000)	[5]	(45,142,000)	[5]
Interest and dividends	[2],[4]	174,913,000		108,009,000		49,440,000
Reimbursement from affiliates	[6]	1,148,000		1,307,000		3,157,000
Aircraft lease revenue	[5]			1,852,000		3,751,000
Reinsurance premiums	[7]	46,335,000		38,096,000		30,996,000
Other revenues	[7]	(396,000)		5,672,000		5,330,000
Total revenues		97,870,000		50,452,000		(10,093,000)
Interest expense	[4]	140,603,000	[2]	81,103,000		42,061,000
Total net revenues		(42,733,000)		(30,651,000)		(52,154,000)
Expenses
Non-interest expense	[1],[6],[8],[9]	79,286,000		76,970,000		58,935,000
Goodwill Impairment		4,100,000
Total expenses		83,386,000		76,970,000		58,935,000
Other income (loss)
Total other income (loss)	[3],[4],[9]	102,109,000		88,248,000	[10]	74,706,000	[10],[11]
Income tax expense (benefit)	[8]	14,853,000		15,719,000		44,053,000
(Net Income (Loss) Attributable to Noncontrolling Interest		8,060,000		1,777,000		3,719,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(30,803,000)		(33,315,000)		(76,717,000)
Preferred stock dividends		0		0
Net income (loss) attributable to Cowen Inc. common stockholders		(30,803,000)		(33,315,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0		0
Expenses
Total expenses		$ 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 fund start-up costs and 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 ) for which the majority of this activity is shown in other income (loss) for US GAAP reporting.

[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. The costs are recorded within expenses for US GAAP reporting.

[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.

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