Segment Reporting (Details)		3 Months Ended
		Mar. 31, 2020 USD ($) customer segment		Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment		2
Entity-wide revenue, major customer, number | customer		0
Revenues
Investment banking		$ 105,028,000		$ 80,106,000
Brokerage		139,362,000		97,463,000
Management fees		11,604,000		7,141,000
Incentive income		0		15,000
Investment Income		0		0
Interest and dividends		42,077,000		29,092,000
Reimbursement from affiliates		261,000		288,000
Reinsurance premiums		10,471,000		6,591,000
Other revenues		1,850,000		1,061,000
Total revenues		313,809,000		224,097,000
Interest expense		38,792,000		29,084,000
Total net revenues		275,017,000		195,013,000
Expenses
Non-interest expense		228,439,000		221,001,000
Total expenses		231,153,000		222,483,000
Other income (loss)
Total other income (loss)		(117,148,000)		40,942,000
Income tax expense (benefit)		(1,173,000)		3,177,000
Net Income (Loss) Attributable to Noncontrolling Interest		(62,188,000)		512,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(9,923,000)		9,783,000
Preferred stock dividends		1,698,000		1,698,000
Net income (loss) attributable to Cowen Inc. common stockholders		(11,621,000)		8,085,000
Consolidated Funds
Revenues
Total revenues		3,156,000		2,340,000
Expenses
Total expenses		2,714,000		1,482,000
Other income (loss)
Net Income (Loss) Attributable to Noncontrolling Interest		(62,315,000)		400,000
Operating Company
Revenues
Management fees		11,346,000		6,643,000
Incentive income		0		0
Asset Company
Revenues
Management fees		258,000		498,000
Incentive income		0		15,000
Operating Segments
Revenues
Investment banking		98,759,000		82,991,000
Brokerage		132,672,000		111,872,000
Management fees		14,915,000		10,431,000
Incentive income		(4,889,000)		16,747,000
Investment Income		(31,104,000)		10,269,000
Interest and dividends		0		0
Reimbursement from affiliates		0		0
Reinsurance premiums		0		0
Other revenues		562,000		1,159,000
Total revenues		210,915,000		233,469,000
Interest expense		7,778,000		6,564,000
Total net revenues		203,137,000		226,905,000
Expenses
Non-interest expense		211,925,000		208,910,000
Total expenses		211,925,000		208,910,000
Other income (loss)
Total other income (loss)		0		0
Income tax expense (benefit)		0		0
Net Income (Loss) Attributable to Noncontrolling Interest		1,740,000		1,023,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(10,528,000)		16,972,000
Preferred stock dividends		1,698,000		1,698,000
Net income (loss) attributable to Cowen Inc. common stockholders		(12,226,000)		15,274,000
Other Depreciation and Amortization		5,434,000		4,956,000
Economic Operating Income		(6,792,000)		20,230,000
Operating Segments | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Operating Segments | Operating Company
Revenues
Investment banking		98,759,000		82,991,000
Brokerage		132,672,000		111,872,000
Management fees		14,711,000		9,728,000
Incentive income		(2,487,000)		16,637,000
Investment Income		(19,449,000)		9,427,000
Interest and dividends		0		0
Reimbursement from affiliates		0		0
Reinsurance premiums		0		0
Other revenues		562,000		1,123,000
Total revenues		224,768,000		231,778,000
Interest expense		6,343,000		5,317,000
Total net revenues		218,425,000		226,461,000
Expenses
Non-interest expense		211,066,000		206,531,000
Total expenses		211,066,000		206,531,000
Other income (loss)
Total other income (loss)		0		0
Income tax expense (benefit)		0		0
Net Income (Loss) Attributable to Noncontrolling Interest		1,740,000		1,023,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		5,619,000		18,907,000
Preferred stock dividends		1,358,000		1,375,000
Net income (loss) attributable to Cowen Inc. common stockholders		4,261,000		17,532,000
Other Depreciation and Amortization		5,428,000		4,939,000
Economic Operating Income		9,689,000		22,471,000
Operating Segments | Operating Company | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		0		0
Operating Segments | Asset Company
Revenues
Investment banking		0		0
Brokerage		0		0
Management fees		204,000		703,000
Incentive income		(2,402,000)		110,000
Investment Income		(11,655,000)		842,000
Interest and dividends		0		0
Reimbursement from affiliates		0		0
Reinsurance premiums		0		0
Other revenues		0		36,000
Total revenues		(13,853,000)		1,691,000
Interest expense		1,435,000		1,247,000
Total net revenues		(15,288,000)		444,000
Expenses
Non-interest expense		859,000		2,379,000
Total expenses		859,000		2,379,000
Other income (loss)
Total other income (loss)		0		0
Income tax expense (benefit)		0		0
Net Income (Loss) Attributable to Noncontrolling Interest		0		0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(16,147,000)		(1,935,000)
Preferred stock dividends		340,000		323,000
Net income (loss) attributable to Cowen Inc. common stockholders		(16,487,000)		(2,258,000)
Other Depreciation and Amortization		6,000		17,000
Economic Operating Income		(16,481,000)		(2,241,000)
Operating Segments | Asset Company | 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		(678,000)		(501,000)
Incentive income		0		(544,000)
Investment Income		0		0
Interest and dividends		0		0
Reimbursement from affiliates		(25,000)		(34,000)
Reinsurance premiums		0		0
Other revenues		0		0
Total revenues		2,453,000		1,261,000
Interest expense		0		0
Total net revenues		2,453,000		1,261,000
Expenses
Non-interest expense		0		0
Total expenses		2,714,000		1,482,000
Other income (loss)
Total other income (loss)		(62,054,000)		621,000
Income tax expense (benefit)		0		0
Net Income (Loss) Attributable to Noncontrolling Interest		(62,315,000)		400,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		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		3,156,000		2,340,000
Expenses
Total expenses		2,714,000		1,482,000
Adjustments | Other Adjustments
Revenues
Investment banking	[1]	6,269,000		(2,885,000)
Brokerage	[2],[3]	6,690,000		(14,409,000)
Management fees	[4]	(2,633,000)		(2,789,000)
Incentive income	[4]	4,889,000		(16,188,000)
Investment Income	[3],[5]	31,104,000		(10,269,000)
Interest and dividends	[2],[5]	42,077,000		29,092,000
Reimbursement from affiliates	[6]	286,000		322,000
Reinsurance premiums	[7]	10,471,000		6,591,000
Other revenues	[7]	1,288,000		(98,000)
Total revenues		100,441,000		(10,633,000)
Interest expense	[5]	31,014,000	[2]	22,520,000
Total net revenues		69,427,000		(33,153,000)
Expenses
Non-interest expense	[1],[6],[8],[9]	16,514,000		12,091,000
Total expenses		16,514,000		12,091,000
Other income (loss)
Total other income (loss)	[4],[5],[9]	(55,094,000)		40,321,000
Income tax expense (benefit)	[8]	(1,173,000)		3,177,000
Net Income (Loss) Attributable to Noncontrolling Interest		(1,613,000)		(911,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		605,000		(7,189,000)
Preferred stock dividends		0		0
Net income (loss) attributable to Cowen Inc. common stockholders		605,000		(7,189,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0
Expenses
Total expenses		$ 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 gains and losses on investments held as part of the Company's facilitation and trading business within brokerage revenues as these investments are directly related to the markets business activities.

[4] 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.

[5] 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.

[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.