Segment Reporting (Details)		3 Months Ended								12 Months Ended
		Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) segment customer		Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Number | customer										0
Number of Operating Segments | segment										2
Revenues
Investment banking										$ 133,279,000		$ 222,781,000		$ 170,506,000
Brokerage										199,180,000		157,722,000		140,132,000
Management fees										40,612,000		41,906,000		40,627,000
Incentive income										8,334,000		1,466,000		2,785,000
Investment Income										0		0		0
Interest and dividends										14,732,000		13,796,000		48,870,000
Aircraft lease revenue										4,161,000		0		0
Reimbursement from affiliates										10,504,000		21,557,000		12,495,000
Reinsurance premiums										32,459,000		0		0
Other revenue										22,355,000		3,726,000		9,446,000
Total revenues		$ 122,268,000	$ 131,027,000	$ 117,231,000	$ 101,039,000	$ 110,611,000	$ 113,254,000	$ 119,608,000	$ 121,094,000	471,565,000		464,567,000		427,776,000
Expenses
Non-interest expense										508,746,000		475,844,000		443,471,000
Goodwill Impairment										0		0		2,334,000
Interest and dividends										29,308,000		26,220,000		42,752,000
Total expenses										547,118,000		504,374,000		490,191,000
Other income (loss)
Total other income (loss)										44,066,000		51,286,000		120,251,000
Income tax expense (benefit)		(12,539,000)	8,759,000	(11,992,000)	(3,320,000)	(52,708,000)	(5,081,000)	3,346,000	6,947,000	(19,092,000)		(47,496,000)		(124,944,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(6,882,000)		(15,246,000)		(15,564,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.		$ (1,904,000)	$ (3,219,000)	$ (10,456,000)	$ (3,698,000)	$ 30,661,000	$ (10,346,000)	$ 6,716,000	$ 16,698,000	(19,277,000)		43,729,000		167,216,000
Consolidated Funds
Revenues
Total revenues										5,949,000		1,613,000		2,915,000
Expenses
Total expenses										9,064,000		2,310,000		1,634,000
Alternative Investment
Expenses
Goodwill Impairment										0		0
Broker-Dealer
Expenses
Goodwill Impairment										0		0
Operating Segments
Revenues
Investment banking										133,279,000		222,781,000		170,506,000
Brokerage										207,040,000		160,436,000		146,247,000
Management fees										67,248,000		70,015,000		64,774,000
Incentive income										26,274,000		(1,544,000)		45,708,000
Investment Income										4,023,000		62,596,000		65,215,000
Interest and dividends										0		0		0
Aircraft lease revenue										0
Reimbursement from affiliates										0		0		0
Reinsurance premiums										0				5,168,000
Other revenue										29,767,000		15,382,000
Total revenues										467,631,000		529,666,000		497,618,000
Expenses
Non-interest expense										471,351,000		469,754,000		435,862,000
Goodwill Impairment														0
Interest and dividends										17,190,000		16,584,000		9,798,000
Total expenses										488,541,000		486,338,000		445,660,000
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										(7,821,000)		(8,796,000)		(7,802,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										(28,731,000)		34,532,000		44,156,000
Operating Segments | Consolidated Funds
Revenues
Other revenue														0
Total revenues										0		0
Expenses
Total expenses										0		0		0
Operating Segments | Alternative Investment
Revenues
Investment banking										0		0		0
Brokerage										0		0		55,000
Management fees										64,086,000		68,989,000		64,774,000
Incentive income										26,274,000		(1,544,000)		45,708,000
Investment Income										3,015,000		49,244,000		45,193,000
Interest and dividends										0		0		0
Aircraft lease revenue										0
Reimbursement from affiliates										0		0		0
Reinsurance premiums										0				4,645,000
Other revenue										29,202,000		14,492,000
Total revenues										122,577,000		131,181,000		160,375,000
Expenses
Non-interest expense										102,163,000		107,291,000		115,601,000
Goodwill Impairment														0
Interest and dividends										12,827,000		11,839,000		7,804,000
Total expenses										114,990,000		119,130,000		123,405,000
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										(7,821,000)		(8,796,000)		(7,802,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										(234,000)		3,255,000		29,168,000
Operating Segments | Alternative Investment | Consolidated Funds
Revenues
Other revenue														0
Total revenues										0		0
Expenses
Total expenses										0		0		0
Operating Segments | Broker-Dealer
Revenues
Investment banking										133,279,000		222,781,000		170,506,000
Brokerage										207,040,000		160,436,000		146,192,000
Management fees										3,162,000		1,026,000		0
Incentive income										0		0		0
Investment Income										1,008,000		13,352,000		20,022,000
Interest and dividends										0		0		0
Aircraft lease revenue										0
Reimbursement from affiliates										0		0		0
Reinsurance premiums										0				523,000
Other revenue										565,000		890,000
Total revenues										345,054,000		398,485,000		337,243,000
Expenses
Non-interest expense										369,188,000		362,463,000		320,261,000
Goodwill Impairment														0
Interest and dividends										4,363,000		4,745,000		1,994,000
Total expenses										373,551,000		367,208,000		322,255,000
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.										(28,497,000)		31,277,000		14,988,000
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										(1,665,000)		(1,307,000)		(963,000)
Incentive income										(714,000)		(736,000)		(281,000)
Investment Income										0		0		0
Interest and dividends										0		0		0
Aircraft lease revenue										0
Reimbursement from affiliates										(303,000)		(190,000)		(342,000)
Reinsurance premiums										0
Other revenue										0		0		0
Total revenues										3,267,000		(620,000)		1,329,000
Expenses
Non-interest expense										(429,000)		0		0
Goodwill Impairment														0
Interest and dividends										0		0		0
Total expenses										8,635,000		2,310,000		1,634,000
Other income (loss)
Total other income (loss)										8,532,000		8,781,000		5,775,000
Income tax expense (benefit)										0		0		0
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										(3,164,000)		(5,851,000)		(5,470,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										0		0		0
Adjustments | Funds Consolidation | Consolidated Funds
Revenues
Total revenues										5,949,000		1,613,000		2,915,000
Expenses
Total expenses										9,064,000		2,310,000		1,634,000
Adjustments | Other Adjustments
Revenues
Investment banking										0		0		0
Brokerage										(7,860,000)		(2,714,000)		(6,115,000)
Management fees	[1]									(24,971,000)		(26,802,000)		(23,184,000)
Incentive income	[1]									(17,226,000)		3,746,000		(42,642,000)
Investment Income	[2]									(4,023,000)		(62,596,000)		(65,215,000)
Interest and dividends	[2]									14,732,000		13,796,000		48,870,000
Aircraft lease revenue	[3]									4,161,000
Reimbursement from affiliates	[4]									10,807,000		21,747,000		12,837,000
Reinsurance premiums	[5]									32,459,000
Other revenue										(7,412,000)	[3],[5]	(11,656,000)	[5]	4,278,000	[2]
Total revenues										667,000		(64,479,000)		(71,171,000)
Expenses
Non-interest expense	[2],[6],[7]									37,824,000		6,090,000		7,609,000
Goodwill Impairment	[8]													2,334,000
Interest and dividends	[2]									12,118,000		9,636,000		32,954,000
Total expenses										49,942,000		15,726,000		42,897,000
Other income (loss)
Total other income (loss)	[2]									35,534,000		42,505,000		114,476,000
Income tax expense (benefit)	[6]									(19,092,000)		(47,496,000)		(124,944,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries and funds										4,103,000		(599,000)		(2,292,000)
Economic Income (Loss) / Net income (loss) attributable to Cowen Group, Inc.										9,454,000		9,197,000		123,060,000
Adjustments | Other Adjustments | Consolidated Funds
Revenues
Total revenues										0		0		0
Expenses
Total expenses										$ 0		$ 0		$ 0

[1] 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 and the activist business.

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

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

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

[5] Economic Income (Loss) recognizes underwriting income from the Company's insurance related activities net of expenses.

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

[7] Economic Income (Loss) recognizes the Company's proportionate share of expenses for certain real estate and other operating entities for which the investments are recorded under the equity method of accounting for investments.

[8] Economic Income (Loss) excludes goodwill impairment and other reorganization expenses.