Segment Reporting (Details) (USD $)	3 Months Ended					6 Months Ended
	Jun. 30, 2014 customer		Jun. 30, 2013		Mar. 31, 2013 customer	Jun. 30, 2014 segment customer		Jun. 30, 2013
Segment Reporting Information [Line Items]
Number of Operating Segments						2
Revenues
Investment banking	$ 30,292,000		$ 25,571,000			$ 79,854,000		$ 42,737,000
Brokerage	33,311,000		31,521,000			66,141,000		58,121,000
Management fees	9,692,000		9,698,000			18,616,000		19,191,000
Incentive income	2,724,000		1,954,000			5,222,000		4,565,000
Interest and dividends	12,460,000		10,521,000			21,712,000		19,842,000
Reimbursement from affiliates	3,018,000		1,214,000			4,918,000		2,699,000
Total revenues	92,902,000		81,207,000			199,579,000		148,448,000
Expenses
Employee compensation and benefits	64,404,000		47,507,000			131,965,000		91,730,000
Interest and dividends	10,193,000		7,489,000			17,265,000		14,109,000
Total expenses	107,904,000		86,612,000			213,932,000		169,724,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	23,037,000		4,994,000			34,391,000		16,801,000
Total other income (loss)	28,646,000		8,911,000			42,103,000		25,877,000
Income (loss) before income taxes and non-controlling interests	13,644,000		3,506,000			27,750,000		4,601,000
Income tax expense (benefit)	46,000		158,000			125,000		334,000
Economic income (loss)/ net income (loss) before non-controlling interests	13,598,000		3,348,000			27,625,000		4,267,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	8,382,000		1,093,000			18,222,000		(1,483,000)
Number of major customers	0				0	0
Reported Under U.S. GAAP
Revenues
Investment banking	30,292,000		25,571,000			79,854,000		42,737,000
Brokerage	33,311,000		31,521,000			66,141,000		58,121,000
Management fees	9,692,000		9,698,000			18,616,000		19,191,000
Incentive income	2,724,000		1,954,000			5,222,000		4,565,000
Investment Income	0		0			0		0
Interest and dividends	12,460,000		10,521,000			21,712,000		19,842,000
Reimbursement from affiliates	3,018,000		1,214,000			4,918,000		2,699,000
Other revenue	752,000		485,000			1,307,000		963,000
Total revenues	92,902,000		81,207,000			199,579,000		148,448,000
Expenses
Employee compensation and benefits	64,404,000		47,507,000			131,965,000		91,730,000
Non-compensation expenses - Fixed	0		0			0		0
Non-compensation expenses - Variable	0		0			0		0
Non-compensation expenses	32,909,000		31,131,000			64,002,000		62,966,000
Interest and dividends	10,193,000		7,489,000			17,265,000		14,109,000
Reimbursement from affiliates	0		0			0		0
Total expenses	107,904,000		86,612,000			213,932,000		169,724,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	23,037,000		4,994,000			34,391,000		16,801,000
Total other income (loss)	28,646,000		8,911,000			42,103,000		25,877,000
Income (loss) before income taxes and non-controlling interests	13,644,000		3,506,000			27,750,000		4,601,000
Income tax expense (benefit)	46,000		158,000			125,000		334,000
Economic income (loss)/ net income (loss) before non-controlling interests	13,598,000		3,348,000			27,625,000		4,267,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(5,216,000)		(2,255,000)			(9,403,000)		(5,750,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	8,382,000		1,093,000			18,222,000		(1,483,000)
Operating Segments | Reported Under Economic Income/(Loss)
Revenues
Investment banking	30,292,000		25,571,000			79,854,000		42,737,000
Brokerage	35,052,000		33,300,000			69,401,000		61,317,000
Management fees	16,166,000		14,606,000			30,255,000		28,750,000
Incentive income	8,193,000		3,765,000			12,919,000		8,892,000
Investment Income	21,596,000		3,563,000			29,770,000		14,436,000
Interest and dividends	0		0			0		0
Reimbursement from affiliates	0		0			0		0
Other revenue	242,000		278,000			97,000		(163,000)
Total revenues	111,541,000		81,083,000			222,296,000		155,969,000
Expenses
Employee compensation and benefits	63,632,000		47,026,000			130,589,000		90,860,000
Non-compensation expenses - Fixed	23,544,000		22,356,000			46,359,000		44,760,000
Non-compensation expenses - Variable	11,136,000		9,989,000			20,573,000		18,714,000
Non-compensation expenses	0		0			0		0
Interest and dividends	2,654,000		72,000			3,297,000		196,000
Reimbursement from affiliates	(1,756,000)		(1,411,000)			(3,482,000)		(2,830,000)
Total expenses	99,210,000		78,032,000			197,336,000		151,700,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Total other income (loss)	0		0			0		0
Income (loss) before income taxes and non-controlling interests	12,331,000		3,051,000			24,960,000		4,269,000
Income tax expense (benefit)	0		0			0		0
Economic income (loss)/ net income (loss) before non-controlling interests	12,331,000		3,051,000			24,960,000		4,269,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(3,818,000)		(1,581,000)			(6,444,000)		(4,067,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	8,513,000		1,470,000			18,516,000		202,000
Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Investment banking	0		0			0		0
Brokerage	25,000		0			25,000		0
Management fees	16,166,000		14,606,000			30,255,000		28,750,000
Incentive income	8,193,000		3,765,000			12,919,000		8,892,000
Investment Income	16,632,000		3,834,000			24,961,000		12,138,000
Interest and dividends	0		0			0		0
Reimbursement from affiliates	0		0			0		0
Other revenue	140,000		114,000			97,000		226,000
Total revenues	41,156,000		22,319,000			68,257,000		50,006,000
Expenses
Employee compensation and benefits	18,666,000		9,723,000			32,053,000		22,949,000
Non-compensation expenses - Fixed	10,228,000		8,471,000			19,122,000		17,609,000
Non-compensation expenses - Variable	1,732,000		1,139,000			2,481,000		2,145,000
Non-compensation expenses	0		0			0		0
Interest and dividends	2,622,000		50,000			3,231,000		131,000
Reimbursement from affiliates	(1,756,000)		(1,411,000)			(3,482,000)		(2,830,000)
Total expenses	31,492,000		17,972,000			53,405,000		40,004,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Total other income (loss)	0		0			0		0
Income (loss) before income taxes and non-controlling interests	9,664,000		4,347,000			14,852,000		10,002,000
Income tax expense (benefit)	0		0			0		0
Economic income (loss)/ net income (loss) before non-controlling interests	9,664,000		4,347,000			14,852,000		10,002,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(3,818,000)		(1,581,000)			(6,444,000)		(4,067,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	5,846,000		2,766,000			8,408,000		5,935,000
Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Investment banking	30,292,000	[1]	25,571,000	[2]		79,854,000	[3]	42,737,000	[4]
Brokerage	35,027,000	[1]	33,300,000	[2]		69,376,000	[3]	61,317,000	[4]
Management fees	0	[1]	0	[2]		0	[3]	0	[4]
Incentive income	0	[1]	0	[2]		0	[3]	0	[4]
Investment Income	4,964,000	[1]	(271,000)	[2]		4,809,000	[3]	2,298,000	[4]
Interest and dividends	0	[1]	0	[2]		0	[3]	0	[4]
Reimbursement from affiliates	0	[1]	0	[2]		0	[3]	0	[4]
Other revenue	102,000	[1]	164,000	[2]		0	[3]	(389,000)	[4]
Total revenues	70,385,000	[1]	58,764,000	[2]		154,039,000	[3]	105,963,000	[4]
Expenses
Employee compensation and benefits	44,966,000	[1]	37,303,000	[2]		98,536,000	[4]	67,911,000	[4]
Non-compensation expenses - Fixed	13,316,000	[1]	13,885,000	[2]		27,237,000	[4]	27,151,000	[4]
Non-compensation expenses - Variable	9,404,000	[1]	8,850,000	[2]		18,092,000	[4]	16,569,000	[4]
Non-compensation expenses	0	[1]	0	[2]		0	[4]	0	[4]
Interest and dividends	32,000	[1]	22,000	[2]		66,000	[4]	65,000	[4]
Reimbursement from affiliates	0	[1]	0	[2]		0	[4]	0	[4]
Total expenses	67,718,000	[1]	60,060,000	[2]		143,931,000	[4]	111,696,000	[4]
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0	[1]	0	[2]		0	[3]	0	[4]
Total other income (loss)	0	[1]	0	[2]		0	[3]	0	[4]
Income (loss) before income taxes and non-controlling interests	2,667,000	[1]	(1,296,000)	[2]		10,108,000	[3]	(5,733,000)	[4]
Income tax expense (benefit)	0	[1]	0	[2]		0	[3]	0	[4]
Economic income (loss)/ net income (loss) before non-controlling interests	2,667,000	[1]	(1,296,000)	[2]		10,108,000	[3]	(5,733,000)	[4]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	0	[1]	0	[2]		0	[3]	0	[4]
Net income (loss) attributable to Cowen Group, Inc. stockholders	2,667,000	[1]	(1,296,000)	[2]		10,108,000	[3]	(5,733,000)	[4]
Investment Income from Entity's Capital	5,700,000		300,000			5,600,000		2,600,000
Compensation Expense Related to Investment of Entity's Capital	1,900,000		100,000			1,800,000		900,000
Significant Reconciling Items | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking	0		0			0		0
Brokerage	(1,741,000)	[5]	(1,779,000)	[5]		(3,260,000)	[5]	(3,196,000)	[5]
Management fees	(6,231,000)	[6]	(4,622,000)	[6]		(11,162,000)	[6]	(8,962,000)	[6]
Incentive income	(5,315,000)	[6]	(1,811,000)	[6]		(7,543,000)	[6]	(4,327,000)	[6]
Investment Income	(21,596,000)	[7]	(3,563,000)	[7]		(29,770,000)	[7]	(14,436,000)	[7]
Interest and dividends	12,460,000	[5],[7]	10,521,000	[5],[7]		21,712,000	[5],[7]	19,842,000	[5],[7]
Reimbursement from affiliates	3,102,000	[8]	1,410,000	[8]		5,082,000	[8]	2,830,000	[8]
Other revenue	510,000	[7]	207,000	[7]		1,210,000	[7]	1,126,000	[7]
Total revenues	(18,811,000)		363,000			(23,731,000)		(7,123,000)
Expenses
Employee compensation and benefits	772,000		481,000			1,376,000		870,000
Non-compensation expenses - Fixed	(23,544,000)	[7],[9]	(22,356,000)	[7],[9]		(46,359,000)	[7],[9]	(44,760,000)	[7],[9]
Non-compensation expenses - Variable	(11,136,000)	[7],[9]	(9,989,000)	[7],[9]		(20,573,000)	[7],[9]	(18,714,000)	[7],[9]
Non-compensation expenses	32,909,000	[7],[9]	31,131,000	[7],[9]		64,002,000	[7],[9]	62,966,000	[7],[9]
Interest and dividends	7,539,000	[5],[7]	7,417,000	[5],[7]		13,968,000	[5],[7]	13,913,000	[5],[7]
Reimbursement from affiliates	1,756,000	[8]	1,411,000	[8]		3,482,000	[8]	2,830,000	[8]
Total expenses	8,296,000		8,095,000			15,896,000		17,105,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	23,037,000	[7]	4,994,000	[7]		34,391,000	[7]	16,801,000	[7]
Total other income (loss)	26,232,000		7,533,000			39,482,000		22,206,000
Income (loss) before income taxes and non-controlling interests	(875,000)		(199,000)			(145,000)		(2,022,000)
Income tax expense (benefit)	46,000	[10]	158,000	[10]		125,000	[10]	334,000	[10]
Economic income (loss)/ net income (loss) before non-controlling interests	(921,000)		(357,000)			(270,000)		(2,356,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	790,000		(20,000)			(24,000)		671,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	(131,000)		(377,000)			(294,000)		(1,685,000)
Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking	0		0			0		0
Brokerage	0		0			0		0
Management fees	(243,000)		(286,000)			(477,000)		(597,000)
Incentive income	(154,000)		0			(154,000)		0
Investment Income	0		0			0		0
Interest and dividends	0		0			0		0
Reimbursement from affiliates	(84,000)		(196,000)			(164,000)		(131,000)
Other revenue	0		0			0		0
Total revenues	172,000		(239,000)			1,014,000		(398,000)
Expenses
Employee compensation and benefits	0		0			0		0
Non-compensation expenses - Fixed	0		0			0		0
Non-compensation expenses - Variable	0		0			0		0
Non-compensation expenses	0		0			0		0
Interest and dividends	0		0			0		0
Reimbursement from affiliates	0		0			0		0
Total expenses	398,000		485,000			700,000		919,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Total other income (loss)	2,414,000		1,378,000			2,621,000		3,671,000
Income (loss) before income taxes and non-controlling interests	2,188,000		654,000			2,935,000		2,354,000
Income tax expense (benefit)	0		0			0		0
Economic income (loss)/ net income (loss) before non-controlling interests	2,188,000		654,000			2,935,000		2,354,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(2,188,000)		(654,000)			(2,935,000)		(2,354,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	0		0			0		0
Consolidated Funds | Reported Under U.S. GAAP
Revenues
Total revenues	653,000		243,000			1,809,000		330,000
Expenses
Total expenses	398,000		485,000			700,000		919,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	5,609,000		3,917,000			7,712,000		9,076,000
Consolidated Funds | Operating Segments | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0		0			0		0
Expenses
Total expenses	0		0			0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Consolidated Funds | Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0		0			0		0
Expenses
Total expenses	0		0			0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0		0			0		0
Consolidated Funds | Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0	[1]	0	[2]		0	[3]	0	[4]
Expenses
Total expenses	0	[1]	0	[2]		0	[4]	0	[4]
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	0	[1]	0	[2]		0	[3]	0	[4]
Consolidated Funds | Significant Reconciling Items | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues	0		0			0		0
Expenses
Total expenses	0		0			0		0
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	3,195,000		2,539,000			5,091,000		5,405,000
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues	653,000		243,000			1,809,000		330,000
Expenses
Total expenses	398,000		485,000			700,000		919,000
Other income (loss)
Net gain (losses) on securities, derivatives and other investments	$ 2,414,000		$ 1,378,000			$ 2,621,000		$ 3,671,000

[1] For the three months ended June 30, 2014, the Company has reflected $5.7 million of investment income and related compensation expense of $1.9 million within the broker-dealer segment in proportion to that segment's capital.

[2] For the three months ended June 30, 2013, the Company has reflected $0.3 million of investment income and related compensation expense of $0.1 million within the broker-dealer segment in proportion to that segment's capital.

[3] For the six months ended June 30, 2014, the Company has reflected $5.6 million of investment income and related compensation expense of $1.8 million within the broker-dealer segment in proportion to that segment's capital.

[4] For the six months ended June 30, 2013, the Company has reflected $2.6 million of investment income and related compensation expense of $0.9 million within the broker-dealer segment in proportion to that segment's capital.

[5] Economic Income (Loss) recognizes stock borrow/loan activity and other brokerage dividends as brokerage revenue

[6] 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.

[7] Economic Income (Loss) recognizes Company income from proprietary trading net of related expenses.

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

[9] 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.

[10] Economic Income (Loss) excludes income taxes as management does not consider this item when evaluating the performance of the segment.