Segment Reporting (Details)		3 Months Ended				9 Months Ended
		Sep. 30, 2018 USD ($)		Sep. 30, 2017 USD ($)		Sep. 30, 2018 USD ($) segment customer		Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment						2
Entity-wide revenue, major customer, number | customer						0
Revenues
Investment banking		$ 81,923,000		$ 57,383,000		$ 264,737,000		$ 158,082,000
Brokerage		92,035,000		84,220,000		301,053,000		198,599,000
Management fees		7,546,000		8,223,000		22,336,000		25,587,000
Incentive income		33,000		1,945,000		97,000		6,217,000
Investment Income		0		0		0		0
Interest and dividends		26,448,000		14,318,000		77,511,000		27,324,000
Reimbursement from affiliates		353,000		484,000		1,066,000		2,631,000
Aircraft lease revenue		375,000		934,000		1,509,000		3,036,000
Reinsurance premiums		8,378,000		7,186,000		26,251,000		21,957,000
Other revenues		1,634,000		3,402,000		3,846,000		6,147,000
Total revenues		221,028,000		178,830,000		706,985,000		454,331,000
Interest expense		26,000,000		15,132,000		74,846,000		37,273,000
Revenue, Net		195,028,000		163,698,000		632,139,000		417,058,000
Expenses
Non-interest expense		209,462,000		177,504,000		653,044,000		478,033,000
Total expenses		211,149,000		179,216,000		660,311,000		487,456,000
Other income (loss)
Total other income (loss)		41,982,000		27,627,000		122,964,000		124,307,000
Income tax expense (benefit)		5,083,000		2,281,000		15,999,000		3,407,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		5,290,000		5,162,000		41,053,000		35,412,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		15,488,000		4,666,000		37,740,000		15,090,000
Consolidated Funds
Revenues
Total revenues		2,303,000		735,000		8,579,000		4,751,000
Expenses
Total expenses		1,687,000		1,712,000		7,267,000		9,423,000
Operating Segments
Revenues
Investment banking		77,510,000		57,383,000		251,476,000		158,082,000
Brokerage		101,672,000		89,968,000		328,929,000		209,274,000
Management fees		12,386,000		13,794,000		37,998,000		42,095,000
Incentive income		6,910,000		4,597,000		21,470,000		18,626,000
Investment Income		27,672,000		15,949,000		61,927,000		51,765,000
Interest and dividends		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenues		83,000		939,000		250,000		3,314,000
Total revenues		226,233,000		182,630,000		702,050,000		483,156,000
Interest expense		6,419,000		4,785,000		16,500,000		13,453,000
Revenue, Net		219,814,000		177,845,000		685,550,000		469,703,000
Expenses
Non-interest expense		195,008,000		169,065,000		610,306,000		440,316,000
Total expenses		195,008,000		169,065,000		610,306,000		440,316,000
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		2,122,000		524,000		6,760,000		4,129,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		22,684,000		8,256,000		68,484,000		25,258,000
Operating Segments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		0		0		0		0
Operating Segments | Investment Management
Revenues
Investment banking		0		0		0		0
Brokerage		0		19,000		0		38,000
Management fees		11,582,000		12,983,000		35,695,000		39,730,000
Incentive income		6,910,000		4,597,000		21,470,000		18,626,000
Investment Income		2,039,000		9,927,000		30,826,000		34,412,000
Interest and dividends		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenues		(283,000)		387,000		(501,000)		2,067,000
Total revenues		20,248,000		27,913,000		87,490,000		94,873,000
Interest expense		2,554,000		3,662,000		8,814,000		10,141,000
Revenue, Net		17,694,000		24,251,000		78,676,000		84,732,000
Expenses
Non-interest expense		17,934,000		21,333,000		67,489,000		68,524,000
Total expenses		17,934,000		21,333,000		67,489,000		68,524,000
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		2,122,000		524,000		6,760,000		4,129,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(2,362,000)		2,394,000		4,427,000		12,079,000
Operating Segments | Investment Management | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		0		0		0		0
Operating Segments | Investment Bank
Revenues
Investment banking		77,510,000		57,383,000		251,476,000		158,082,000
Brokerage		101,672,000		89,949,000		328,929,000		209,236,000
Management fees		804,000		811,000		2,303,000		2,365,000
Incentive income		0		0		0		0
Investment Income		25,633,000		6,022,000		31,101,000		17,353,000
Interest and dividends		0		0		0		0
Reimbursement from affiliates		0		0		0		0
Aircraft lease revenue		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenues		366,000		552,000		751,000		1,247,000
Total revenues		205,985,000		154,717,000		614,560,000		388,283,000
Interest expense		3,865,000		1,123,000		7,686,000		3,312,000
Revenue, Net		202,120,000		153,594,000		606,874,000		384,971,000
Expenses
Non-interest expense		177,074,000		147,732,000		542,817,000		371,792,000
Total expenses		177,074,000		147,732,000		542,817,000		371,792,000
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		0		0		0		0
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		25,046,000		5,862,000		64,057,000		13,179,000
Operating Segments | Investment Bank | 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		(381,000)		(640,000)		(2,197,000)		(1,773,000)
Incentive income		(20,000)		(402,000)		(37,000)		(3,669,000)
Investment Income		0		0		0		0
Interest and dividends		0		0		0		0
Reimbursement from affiliates		(65,000)		(88,000)		(198,000)		(241,000)
Aircraft lease revenue		0		0		0		0
Reinsurance premiums		0		0		0		0
Other revenues		0		0		0		0
Total revenues		1,837,000		(395,000)		6,147,000		(932,000)
Interest expense		0		0		0		0
Revenue, Net		1,837,000		(395,000)		6,147,000		(932,000)
Expenses
Non-interest expense		0		0		0		(199,000)
Total expenses		1,687,000		1,712,000		7,267,000		9,224,000
Other income (loss)
Total other income (loss)		2,654,000		7,535,000		36,113,000		44,111,000
Income tax expense (benefit)		0		0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		2,804,000		5,428,000		34,993,000		33,955,000
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		0		0		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues		2,303,000		735,000		8,579,000		4,751,000
Expenses
Total expenses		1,687,000		1,712,000		7,267,000		9,423,000
Adjustments | Other Adjustments
Revenues
Investment banking		4,413,000	[1]	0		13,261,000	[1]	0
Brokerage	[2]	(9,637,000)		(5,748,000)		(27,876,000)		(10,675,000)
Management fees	[3]	(4,459,000)		(4,931,000)		(13,465,000)		(14,735,000)
Incentive income	[3]	(6,857,000)	[1]	(2,250,000)		(21,336,000)		(8,740,000)
Investment Income	[4],[5]	(27,672,000)		(15,949,000)		(61,927,000)		(51,765,000)
Interest and dividends	[5]	26,448,000		14,318,000		77,511,000		27,324,000
Reimbursement from affiliates	[6]	418,000		572,000		1,264,000		2,872,000
Aircraft lease revenue	[4]	375,000		934,000		1,509,000		3,036,000
Reinsurance premiums	[7]	8,378,000		7,186,000		26,251,000		21,957,000
Other revenues	[7]	1,551,000		2,463,000		3,596,000		2,833,000
Total revenues		(7,042,000)		(3,405,000)		(1,212,000)	[5]	(27,893,000)
Interest expense	[5]	19,581,000		10,347,000		58,346,000		23,820,000
Revenue, Net		(26,623,000)		(13,752,000)		(59,558,000)		(51,713,000)
Expenses
Non-interest expense	[8],[9]	14,454,000	[1],[5]	8,439,000	[5]	42,738,000	[1]	37,916,000
Total expenses		14,454,000		8,439,000		42,738,000		37,916,000
Other income (loss)
Total other income (loss)	[5]	39,328,000	[10]	20,092,000	[11]	86,851,000	[10]	80,196,000	[11]
Income tax expense (benefit)	[8]	5,083,000		2,281,000		15,999,000		3,407,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds		364,000		(790,000)		(700,000)		(2,672,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		(7,196,000)		(3,590,000)		(30,744,000)		(10,168,000)
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues		0		0		0		0
Expenses
Total expenses		$ 0		$ 0		$ 0		$ 0

[1] Economic Income (Loss) presents underwriting expenses net of investment banking revenues 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.