Segment Reporting (Details)		3 Months Ended				6 Months Ended				12 Months Ended
		Jun. 30, 2019 USD ($)		Jun. 30, 2018 USD ($)		Jun. 30, 2019 USD ($) segment customer		Jun. 30, 2018 USD ($)		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		$ 114,705,000		$ 84,826,000		$ 194,811,000		$ 182,814,000
Brokerage		111,382,000		103,285,000		208,845,000		209,018,000
Management fees		7,039,000		7,373,000		14,180,000		14,790,000
Incentive income		8,000		48,000		23,000		64,000
Investment Income		0		0		0		0
Interest and dividends		40,047,000		25,109,000		69,139,000		51,063,000
Reimbursement from affiliates		254,000		336,000		542,000		713,000
Aircraft lease revenue		0		419,000		0		1,134,000
Reinsurance premiums		14,331,000		9,226,000		20,922,000		17,873,000
Other revenues		930,000		876,000		1,991,000		2,212,000
Total revenues		292,164,000		234,573,000		516,261,000		485,957,000
Interest expense		39,528,000		24,306,000		68,612,000		48,846,000
Total net revenues		252,636,000		210,267,000		447,649,000		437,111,000
Expenses
Non-interest expense		240,781,000		220,267,000		460,088,000		443,582,000
Goodwill, Impairment Loss		4,100,000	[1],[2],[3],[4]	0		4,100,000		0		$ 0
Total expenses		247,112,000		223,416,000		467,901,000		449,162,000
Other income (loss)
Total other income (loss)		9,692,000		47,148,000		50,634,000		80,982,000
Income tax expense (benefit)		5,073,000		3,993,000		8,250,000		10,916,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		4,331,000		24,607,000		6,537,000		35,763,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		5,812,000		5,399,000		15,595,000		22,252,000
Preferred stock dividends		1,698,000		1,698,000		3,396,000		3,396,000
Net income (loss) attributable to Cowen Inc. common stockholders		4,114,000		3,701,000		12,199,000		18,856,000
Consolidated Funds
Revenues
Total revenues		3,468,000		3,075,000		5,808,000		6,276,000
Expenses
Total expenses		2,231,000		3,149,000		3,713,000		5,580,000
Operating Company
Revenues
Management fees		6,796,000		6,287,000		13,439,000		12,960,000
Incentive income		8,000		48,000		8,000		64,000
Asset Company
Revenues
Management fees		243,000		1,086,000		741,000		1,830,000
Incentive income		0		0		15,000		0
Operating Segments
Revenues
Investment banking		104,216,000		80,042,000		187,212,000		173,966,000
Brokerage		120,672,000		113,186,000		225,829,000		227,257,000
Management fees		10,468,000		12,486,000		20,898,000		25,613,000
Incentive income		4,229,000		9,363,000		20,976,000		14,559,000
Investment Income		485,000		19,954,000		17,468,000		34,256,000
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		4,319,000		(721,000)		5,478,000		166,000
Total revenues		244,389,000		234,310,000		477,861,000		475,817,000
Interest expense		6,706,000		4,124,000		13,270,000		10,081,000
Total net revenues		237,683,000		230,186,000		464,591,000		465,736,000
Expenses
Non-interest expense		218,819,000		205,965,000		426,004,000		415,298,000
Goodwill, Impairment Loss		0				0
Total expenses		218,819,000		205,965,000		426,004,000		415,298,000
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		1,683,000		2,480,000		4,431,000		4,638,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		17,181,000		21,741,000		34,156,000		45,800,000
Preferred stock dividends		1,698,000		1,698,000		3,396,000		3,396,000
Net income (loss) attributable to Cowen Inc. common stockholders		15,483,000		20,043,000		30,760,000		42,404,000
Other Depreciation and Amortization		4,952,000		2,959,000		9,908,000		5,945,000
Economic Operating Income		20,435,000		23,002,000		40,668,000		48,349,000
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		104,216,000		80,042,000		187,212,000		173,966,000
Brokerage		120,672,000		113,186,000		225,829,000		227,257,000
Management fees		9,968,000		10,938,000		19,695,000		22,217,000
Incentive income		2,633,000		12,321,000		19,270,000		15,360,000
Investment Income		(3,141,000)		18,476,000		13,001,000		36,710,000
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		4,304,000		(811,000)		5,427,000		(147,000)
Total revenues		238,652,000		234,152,000		470,434,000		475,363,000
Interest expense		5,298,000		3,134,000		10,615,000		7,661,000
Total net revenues		233,354,000		231,018,000		459,819,000		467,702,000
Expenses
Non-interest expense		215,808,000		203,684,000		421,562,000		410,059,000
Goodwill, Impairment Loss		0				0
Total expenses		215,808,000		203,684,000		421,562,000		410,059,000
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		1,448,000		1,672,000		3,250,000		3,812,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		16,098,000		25,662,000		35,007,000		53,831,000
Preferred stock dividends		1,341,000		1,290,000		2,717,000		2,581,000
Net income (loss) attributable to Cowen Inc. common stockholders		14,757,000		24,372,000		32,290,000		51,250,000
Other Depreciation and Amortization		4,945,000		2,913,000		9,884,000		5,842,000
Economic Operating Income		19,702,000		27,285,000		42,174,000		57,092,000
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		500,000		1,548,000		1,203,000		3,396,000
Incentive income		1,596,000		(2,958,000)		1,706,000		(801,000)
Investment Income		3,626,000		1,478,000		4,467,000		(2,454,000)
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		15,000		90,000		51,000		313,000
Total revenues		5,737,000		158,000		7,427,000		454,000
Interest expense		1,408,000		990,000		2,655,000		2,420,000
Total net revenues		4,329,000		(832,000)		4,772,000		(1,966,000)
Expenses
Non-interest expense		3,011,000		2,281,000		4,442,000		5,239,000
Goodwill, Impairment Loss		0				0
Total expenses		3,011,000		2,281,000		4,442,000		5,239,000
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		235,000		808,000		1,181,000		826,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		1,083,000		(3,921,000)		(851,000)		(8,031,000)
Preferred stock dividends		357,000		408,000		679,000		815,000
Net income (loss) attributable to Cowen Inc. common stockholders		726,000		(4,329,000)		(1,530,000)		(8,846,000)
Other Depreciation and Amortization		7,000		46,000		24,000		103,000
Economic Operating Income		733,000		(4,283,000)		(1,506,000)		(8,743,000)
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		(565,000)		(612,000)		(1,066,000)		(1,815,000)
Incentive income		0		(8,000)		(544,000)		(17,000)
Investment Income		0		0		0		0
Interest and dividends		0		0		0		0
Reimbursement from affiliates		(33,000)		(65,000)		(67,000)		(133,000)
Aircraft lease revenue				0				0
Reinsurance premiums		0		0		0		0
Other revenues		12,000		0		12,000		0
Total revenues		2,882,000		2,390,000		4,143,000		4,311,000
Interest expense		0		0		0		0
Total net revenues		2,882,000		2,390,000		4,143,000		4,311,000
Expenses
Non-interest expense		0		0		0		0
Goodwill, Impairment Loss		0				0
Total expenses		2,231,000		3,149,000		3,713,000		5,580,000
Other income (loss)
Total other income (loss)		2,827,000		23,411,000		3,448,000		33,458,000
Income tax expense (benefit)		0		0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		3,478,000		22,652,000		3,878,000		32,189,000
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		3,468,000		3,075,000		5,808,000		6,276,000
Expenses
Total expenses		2,231,000		3,149,000		3,713,000		5,580,000
Adjustments | Other Adjustments
Revenues
Investment banking	[2]	10,489,000		4,784,000		7,599,000		8,848,000
Brokerage	[5]	(9,290,000)		(9,901,000)		(16,984,000)		(18,239,000)
Management fees		(2,864,000)		(4,501,000)	[6]	(5,652,000)	[6]	(9,008,000)	[6]
Incentive income		(4,221,000)		(9,307,000)	[6]	(20,409,000)	[6]	(14,478,000)	[6]
Investment Income	[3]	(485,000)	[7]	(19,954,000)	[8]	(17,468,000)		(34,256,000)	[8]
Interest and dividends	[3]	40,047,000		25,109,000	[5]	69,139,000	[5]	51,063,000	[5]
Reimbursement from affiliates		287,000		401,000	[7]	609,000	[7]	846,000	[7]
Aircraft lease revenue	[8]			419,000				1,134,000
Reinsurance premiums		14,331,000	[8]	9,226,000	[9]	20,922,000	[9]	17,873,000	[9]
Other revenues		(3,401,000)	[8]	1,597,000	[9]	(3,499,000)	[9]	2,046,000	[9]
Total revenues		44,893,000		(2,127,000)		34,257,000		5,829,000
Interest expense	[3]	32,822,000		20,182,000	[5]	55,342,000	[5]	38,765,000	[5]
Total net revenues		12,071,000		(22,309,000)		(21,085,000)		(32,936,000)
Expenses
Non-interest expense	[1],[2],[4]	21,962,000	[3]	14,302,000	[7]	34,084,000	[7]	28,284,000	[7]
Goodwill, Impairment Loss		4,100,000	[1],[2],[3],[4]			4,100,000
Total expenses		26,062,000		14,302,000		38,184,000		28,284,000
Other income (loss)
Total other income (loss)	[3]	6,865,000	[10]	23,737,000	[4],[6],[10]	47,186,000	[4],[6]	47,524,000	[4],[6],[10]
Income tax expense (benefit)	[1]	5,073,000		3,993,000		8,250,000		10,916,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		(830,000)		(525,000)		(1,772,000)		(1,064,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(11,369,000)		(16,342,000)		(18,561,000)		(23,548,000)
Preferred stock dividends		0		0		0		0
Net income (loss) attributable to Cowen Inc. common stockholders		(11,369,000)		(16,342,000)		(18,561,000)		(23,548,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		$ 0		$ 0		$ 0		$ 0

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

[2] Economic Income (Loss) presents underwriting expenses net of investment banking revenues, expenses reimbursedfrom 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

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

[4] 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.

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

[6] 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.

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

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

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

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