Segment Reporting (Details)		3 Months Ended								12 Months Ended
		Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment customer		Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer										0
Number of Operating Segments | segment										2
Revenues
Investment banking										$ 223,614,000		$ 133,279,000		$ 222,781,000
Brokerage										293,610,000		199,180,000		157,722,000
Management fees										33,245,000		40,612,000		41,906,000
Incentive income										5,383,000		8,334,000		1,466,000
Investment Income										0		0		0
Interest and dividends										49,440,000		14,732,000		13,796,000
Aircraft lease revenue										3,751,000		4,161,000		0
Reimbursement from affiliates										2,860,000		10,504,000		21,557,000
Reinsurance premiums										30,996,000		32,459,000		0
Other revenue										8,561,000		22,355,000		3,726,000
Total revenues		$ 204,450,000	$ 178,830,000	$ 160,530,000	$ 114,971,000	$ 122,268,000	$ 131,027,000	$ 117,231,000	$ 101,039,000	658,781,000		471,565,000		464,567,000
Expenses
Non-interest expense										684,123,000		508,746,000		475,844,000
Interest and dividends										60,949,000		29,308,000		26,220,000
Total expenses										757,598,000		547,118,000		504,374,000
Other income (loss)
Total other income (loss)										105,779,000		44,066,000		51,286,000
Income tax expense (benefit)		40,646,000	2,281,000	(785,000)	1,911,000	(12,539,000)	8,759,000	(11,992,000)	(3,320,000)	44,053,000		(19,092,000)		(47,496,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(23,791,000)		(6,882,000)		(15,246,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		$ (75,972,000)	$ 4,666,000	$ 7,440,000	$ 2,984,000	$ (1,904,000)	$ (3,219,000)	$ (10,456,000)	$ (3,698,000)	(60,882,000)		(19,277,000)		43,729,000
Consolidated Funds
Revenues
Total revenues										7,321,000		5,949,000		1,613,000
Expenses
Total expenses										12,526,000		9,064,000		2,310,000
Operating Segments
Revenues
Investment banking										223,614,000		133,279,000		222,781,000
Brokerage										312,780,000		207,040,000		160,436,000
Management fees										55,387,000		67,248,000		70,015,000
Incentive income										26,028,000		26,274,000		(1,544,000)
Investment Income										45,142,000		4,023,000		62,596,000
Interest and dividends										0		0		0
Aircraft lease revenue										0		0
Reimbursement from affiliates										0		0		0
Reinsurance premiums										0		0		15,382,000
Other revenue										3,231,000		29,767,000
Total revenues										666,182,000		467,631,000		529,666,000
Expenses
Non-interest expense										625,387,000		471,351,000		469,754,000
Interest and dividends										18,888,000		17,190,000		16,584,000
Total expenses										644,275,000		488,541,000		486,338,000
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										(6,072,000)		(7,821,000)		(8,796,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										15,835,000		(28,731,000)		34,532,000
Operating Segments | Consolidated Funds
Revenues
Other revenue														0
Total revenues										0		0
Expenses
Total expenses										0		0		0
Operating Segments | Investment Management
Revenues
Investment banking										0		0		0
Brokerage										38,000		0		0
Management fees										52,239,000		64,086,000		68,989,000
Incentive income										26,028,000		26,274,000		(1,544,000)
Investment Income										31,447,000		3,015,000		49,244,000
Interest and dividends										0		0		0
Aircraft lease revenue										0		0
Reimbursement from affiliates										0		0		0
Reinsurance premiums										0		0		14,492,000
Other revenue										1,575,000		29,202,000
Total revenues										111,327,000		122,577,000		131,181,000
Expenses
Non-interest expense										92,567,000		102,163,000		107,291,000
Interest and dividends										14,268,000		12,827,000		11,839,000
Total expenses										106,835,000		114,990,000		119,130,000
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										(6,072,000)		(7,821,000)		(8,796,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										(1,580,000)		(234,000)		3,255,000
Operating Segments | Investment Management | Consolidated Funds
Revenues
Other revenue														0
Total revenues										0		0
Expenses
Total expenses										0		0		0
Operating Segments | Broker-Dealer
Revenues
Investment banking										223,614,000		133,279,000		222,781,000
Brokerage										312,742,000		207,040,000		160,436,000
Management fees										3,148,000		3,162,000		1,026,000
Incentive income										0		0		0
Investment Income										13,695,000		1,008,000		13,352,000
Interest and dividends										0		0		0
Aircraft lease revenue										0		0
Reimbursement from affiliates										0		0		0
Reinsurance premiums										0		0		890,000
Other revenue										1,656,000		565,000
Total revenues										554,855,000		345,054,000		398,485,000
Expenses
Non-interest expense										532,820,000		369,188,000		362,463,000
Interest and dividends										4,620,000		4,363,000		4,745,000
Total expenses										537,440,000		373,551,000		367,208,000
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.										17,415,000		(28,497,000)		31,277,000
Operating Segments | Broker-Dealer | Consolidated Funds
Revenues
Other revenue														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										(2,593,000)		(1,665,000)		(1,307,000)
Incentive income										(1,739,000)		(714,000)		(736,000)
Investment Income										0		0		0
Interest and dividends										0		0		0
Aircraft lease revenue										0		0
Reimbursement from affiliates										(297,000)		(303,000)		(190,000)
Reinsurance premiums										0		0
Other revenue										0		0		0
Total revenues										2,692,000		3,267,000		(620,000)
Expenses
Non-interest expense										(199,000)		(429,000)		0
Interest and dividends										0		0		0
Total expenses										12,327,000		8,635,000		2,310,000
Other income (loss)
Total other income (loss)										31,073,000		8,532,000		8,781,000
Income tax expense (benefit)										0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(21,438,000)		(3,164,000)		(5,851,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										0		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues										7,321,000		5,949,000		1,613,000
Expenses
Total expenses										12,526,000		9,064,000		2,310,000
Adjustments | Other Adjustments
Revenues
Investment banking										0		0		0
Brokerage										(19,170,000)	[1]	(7,860,000)	[1]	(2,714,000)
Management fees	[2]									(19,549,000)		(24,971,000)		(26,802,000)
Incentive income	[2]									(18,906,000)		(17,226,000)		3,746,000
Investment Income	[4]									(45,142,000)	[3]	(4,023,000)	[3]	(62,596,000)
Interest and dividends	[4]									49,440,000		14,732,000		13,796,000
Aircraft lease revenue	[3]									3,751,000		4,161,000
Reimbursement from affiliates	[5]									3,157,000		10,807,000		21,747,000
Reinsurance premiums	[6]									30,996,000		32,459,000
Other revenue										5,330,000	[6]	(7,412,000)	[6]	(11,656,000)	[4]
Total revenues										(10,093,000)		667,000		(64,479,000)
Expenses
Non-interest expense	[7],[8]									58,935,000		37,824,000		6,090,000	[4]
Interest and dividends	[4]									42,061,000		12,118,000		9,636,000
Total expenses										100,996,000		49,942,000		15,726,000
Other income (loss)
Total other income (loss)										74,706,000	[1],[9],[10]	35,534,000	[4]	42,505,000	[4]
Income tax expense (benefit)	[7]									44,053,000		(19,092,000)		(47,496,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										3,719,000		4,103,000		(599,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										(76,717,000)		9,454,000		9,197,000
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues										0		0		0
Expenses
Total expenses										$ 0		$ 0		$ 0

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

[2] 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.

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

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

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

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

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

[8] 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.

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

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