Segment Reporting (Details)		3 Months Ended								12 Months Ended
		Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment customer		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment										2
Entity-wide revenue, major customer, number | customer										0
Revenues
Investment banking										$ 357,222,000		$ 223,614,000		$ 133,279,000
Brokerage										413,582,000		293,610,000		199,180,000
Management fees										29,658,000		33,245,000		40,612,000
Incentive income										3,117,000		5,383,000		8,334,000
Investment Income										0		0		0
Interest and dividends										108,009,000		49,440,000		14,732,000
Reimbursement from affiliates										1,038,000		2,860,000		10,504,000
Aircraft lease revenue										1,852,000		3,751,000		4,161,000
Reinsurance premiums										38,096,000		30,996,000		32,459,000
Other revenues										4,504,000		8,561,000		22,355,000
Total revenues		$ 259,931,000	$ 221,028,000	$ 234,573,000	$ 251,384,000	$ 204,450,000	$ 178,830,000	$ 160,530,000	$ 114,971,000	966,916,000		658,781,000		471,565,000
Interest expense										104,116,000		60,949,000		29,308,000
Total net revenues										862,800,000		597,832,000		442,257,000
Expenses
Non-interest expense										878,991,000		684,123,000		508,746,000
Total expenses										887,606,000		696,649,000		517,810,000
Other income (loss)
Total other income (loss)										123,742,000		105,779,000		44,066,000
Income tax expense (benefit)		(280,000)	5,083,000	3,993,000	6,923,000	40,646,000	2,281,000	(785,000)	1,911,000	15,719,000		44,053,000		(19,092,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										40,398,000		23,791,000		6,882,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		5,079,000	15,488,000	5,399,000	16,853,000	(75,972,000)	4,666,000	7,440,000	2,984,000	42,819,000		(60,882,000)		(19,277,000)
Preferred stock dividends		1,698,000	1,698,000	1,698,000	1,698,000	1,698,000	1,698,000	1,698,000	1,698,000	6,792,000		6,792,000		6,792,000
Net income (loss) attributable to Cowen Inc. common stockholders		$ 3,381,000	$ 13,790,000	$ 3,701,000	$ 15,155,000	$ (77,670,000)	$ 2,968,000	$ 5,742,000	$ 1,286,000	36,027,000		(67,674,000)		(26,069,000)
Other Depreciation and Amortization										7,400,000		7,000,000		7,700,000
Consolidated Funds
Revenues
Total revenues										9,838,000		7,321,000		5,949,000
Expenses
Total expenses										8,615,000		12,526,000		9,064,000
Operating Segments
Revenues
Investment banking										329,061,000		223,614,000		133,279,000
Brokerage										452,299,000		312,780,000		207,040,000
Management fees										49,175,000		55,387,000		67,248,000
Incentive income										23,747,000		26,028,000		26,274,000
Investment Income										56,346,000		45,142,000		4,023,000
Interest and dividends										0		0		0
Reimbursement from affiliates										0		0		0
Aircraft lease revenue										0		0		0
Reinsurance premiums										0		0		0
Other revenues										(1,168,000)		3,231,000		29,767,000
Total revenues										909,460,000		666,182,000		467,631,000
Interest expense										23,013,000		18,888,000		17,190,000
Total net revenues										886,447,000		647,294,000		450,441,000
Expenses
Non-interest expense										802,021,000		625,387,000		471,351,000
Total expenses										802,021,000		625,387,000		471,351,000
Other income (loss)
Total other income (loss)										0		0		0
Income tax expense (benefit)										0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										8,292,000		6,072,000		7,821,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										76,134,000		15,835,000		(28,731,000)
Preferred stock dividends										6,792,000		6,792,000		6,792,000
Net income (loss) attributable to Cowen Inc. common stockholders										69,342,000		9,043,000		(35,523,000)
Other Depreciation and Amortization										11,583,000		11,558,000		11,009,000
Operating Segments | Consolidated Funds
Revenues
Other revenues														0
Total revenues										0		0
Expenses
Total expenses										0		0		0
Operating Segments | Investment Management
Revenues
Investment banking										0		0		0
Brokerage										0		38,000		0
Management fees										46,017,000		52,239,000		64,086,000
Incentive income										23,747,000		26,028,000		26,274,000
Investment Income										37,642,000		31,447,000		3,015,000
Interest and dividends										0		0		0
Reimbursement from affiliates										0		0		0
Aircraft lease revenue										0		0		0
Reinsurance premiums										0		0		0
Other revenues										(2,143,000)		1,575,000		29,202,000
Total revenues										105,263,000		111,327,000		122,577,000
Interest expense										11,396,000		14,268,000		12,827,000
Total net revenues										93,867,000		97,059,000		109,750,000
Expenses
Non-interest expense										83,943,000		92,567,000		102,163,000
Total expenses										83,943,000		92,567,000		102,163,000
Other income (loss)
Total other income (loss)										0		0		0
Income tax expense (benefit)										0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										8,292,000		6,072,000		7,821,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										1,632,000		(1,580,000)		(234,000)
Operating Segments | Investment Management | Consolidated Funds
Revenues
Other revenues														0
Total revenues										0		0
Expenses
Total expenses										0		0		0
Operating Segments | Investment Bank
Revenues
Investment banking										329,061,000		223,614,000		133,279,000
Brokerage										452,299,000		312,742,000		207,040,000
Management fees										3,158,000		3,148,000		3,162,000
Incentive income										0		0		0
Investment Income										18,704,000		13,695,000		1,008,000
Interest and dividends										0		0		0
Reimbursement from affiliates										0		0		0
Aircraft lease revenue										0		0		0
Reinsurance premiums										0		0		0
Other revenues										975,000		1,656,000		565,000
Total revenues										804,197,000		554,855,000		345,054,000
Interest expense										11,617,000		4,620,000		4,363,000
Total net revenues										792,580,000		550,235,000		340,691,000
Expenses
Non-interest expense										718,078,000		532,820,000		369,188,000
Total expenses										718,078,000		532,820,000		369,188,000
Other income (loss)
Total other income (loss)										0		0		0
Income tax expense (benefit)										0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										0		0		0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										74,502,000		17,415,000		(28,497,000)
Operating Segments | Investment Bank | 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										(2,513,000)		(2,593,000)		(1,665,000)
Incentive income										(52,000)		(1,739,000)		(714,000)
Investment Income										0		0		0
Interest and dividends										0		0		0
Reimbursement from affiliates										(269,000)		(297,000)		(303,000)
Aircraft lease revenue										0		0		0
Reinsurance premiums										0		0		0
Other revenues										0		0		0
Total revenues										7,004,000		2,692,000		3,267,000
Interest expense										0		0		0
Total net revenues										7,004,000		2,692,000		3,267,000
Expenses
Non-interest expense										0		(199,000)		(429,000)
Total expenses										8,615,000		12,327,000		8,635,000
Other income (loss)
Total other income (loss)										35,494,000		31,073,000		8,532,000
Income tax expense (benefit)										0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										33,883,000		21,438,000		3,164,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										0		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues										9,838,000		7,321,000		5,949,000
Expenses
Total expenses										8,615,000		12,526,000		9,064,000
Adjustments | Other Adjustments
Revenues
Investment banking										28,161,000	[1]	0		0
Brokerage										(38,717,000)	[2]	(19,170,000)	[2]	(7,860,000)
Management fees	[3]									(17,004,000)		(19,549,000)		(24,971,000)
Incentive income	[3]									(20,578,000)	[1]	(18,906,000)		(17,226,000)
Investment Income	[5]									(56,346,000)	[4]	(45,142,000)	[4]	(4,023,000)
Interest and dividends	[5]									108,009,000		49,440,000		14,732,000
Reimbursement from affiliates	[6]									1,307,000		3,157,000		10,807,000
Aircraft lease revenue	[4]									1,852,000		3,751,000		4,161,000
Reinsurance premiums	[7]									38,096,000		30,996,000		32,459,000
Other revenues	[7]									5,672,000		5,330,000		(7,412,000)
Total revenues										50,452,000		(10,093,000)		667,000
Interest expense	[5]									81,103,000		42,061,000		12,118,000
Total net revenues										(30,651,000)		(52,154,000)		(11,451,000)
Expenses
Non-interest expense	[5],[8],[9]									76,970,000		58,935,000		37,824,000
Total expenses										76,970,000		58,935,000		37,824,000
Other income (loss)
Total other income (loss)	[5]									88,248,000	[10],[11]	74,706,000	[10],[11]	35,534,000
Income tax expense (benefit)	[8]									15,719,000		44,053,000		(19,092,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(1,777,000)		(3,719,000)		(4,103,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										(33,315,000)		(76,717,000)		9,454,000
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues										0		0		0
Expenses
Total expenses										0		0		0
Other Adjustments | Operating Segments
Other income (loss)
Net income (loss) attributable to Cowen Inc. common stockholders										$ 80,925,000		$ 20,601,000		$ (24,514,000)

[1] Economic Income (Loss) presents underwriting expenses net of investment banking revenues, expenses reimbursedfrom clients within their respective expense category and records income from uncrystallized incentive fees.

[2] Economic Income (Loss) brokerage revenues included net securities borrowed and securities loaned activities.

[3] 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.

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

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

[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.

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