Segment Reporting (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 segment		Mar. 31, 2012
Segment Reporting Information [Line Items]
Number of Operating Segments	2
Revenues
Investment banking	$ 17,166,000		$ 15,630,000
Brokerage	26,600,000		24,013,000
Management fees	9,493,000		9,717,000
Incentive income	2,611,000		691,000
Interest and dividends	9,321,000		5,372,000
Reimbursement from affiliates	1,485,000		1,045,000
Expenses
Employee compensation and benefits	44,223,000		46,683,000
Interest and dividends	6,418,000		1,724,000
Total expenses	83,112,000		76,739,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	11,807,000		19,671,000
Total other income (loss)	16,966,000		25,637,000
Income taxes expense / (benefit)	176,000		142,000
Economic income (loss)/ net income (loss) before non-controlling interests	919,000		6,236,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	(2,576,000)		3,995,000
Reported Under U.S. GAAP
Revenues
Investment banking	17,166,000		15,630,000
Brokerage	26,600,000		24,013,000
Management fees	9,493,000		9,717,000
Incentive income	2,611,000		691,000
Investment Income	0		0
Interest and dividends	9,321,000		5,372,000
Reimbursement from affiliates	1,485,000		1,045,000
Other revenue	478,000		867,000
Total revenues	67,241,000		57,480,000
Expenses
Employee compensation and benefits	44,223,000		46,683,000
Interest and dividends	6,418,000		1,724,000
Non-compensation expenses - Fixed	0		0
Non-compensation expenses - Variable	0		0
Non-compensation expenses	32,037,000		27,957,000
Reimbursement from affiliates	0		0
Total expenses	83,112,000		76,739,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	11,807,000		19,671,000
Total other income (loss)	16,966,000		25,637,000
Income (loss) before income taxes and non-controlling interests	1,095,000		6,378,000
Income taxes expense / (benefit)	176,000		142,000
Economic income (loss)/ net income (loss) before non-controlling interests	919,000		6,236,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(3,495,000)		(2,241,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	(2,576,000)		3,995,000
Operating Segments | Reported Under Economic Income/(Loss)
Revenues
Investment banking	17,166,000		15,630,000
Brokerage	28,017,000		24,013,000
Management fees	14,144,000		14,020,000
Incentive income	5,126,000		4,022,000
Investment Income	10,872,000		21,105,000
Interest and dividends	0		0
Reimbursement from affiliates	0		0
Other revenue	(440,000)		384,000
Total revenues	74,885,000		79,174,000
Expenses
Employee compensation and benefits	44,521,000		45,909,000
Interest and dividends	124,000		88,000
Non-compensation expenses - Fixed	23,778,000		21,243,000
Non-compensation expenses - Variable	7,351,000		6,875,000
Non-compensation expenses	0		0
Reimbursement from affiliates	(1,420,000)		(1,116,000)
Total expenses	74,354,000		72,999,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0
Total other income (loss)	0		0
Income (loss) before income taxes and non-controlling interests	531,000		6,175,000
Income taxes expense / (benefit)	0		0
Economic income (loss)/ net income (loss) before non-controlling interests	531,000		6,175,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,800,000)		(300,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	(1,269,000)		5,875,000
Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Investment banking	0		0
Brokerage	0		0
Management fees	14,144,000		14,020,000
Incentive income	5,126,000		4,022,000
Investment Income	8,304,000		16,803,000
Interest and dividends	0		0
Reimbursement from affiliates	0		0
Other revenue	113,000		124,000
Total revenues	27,687,000		34,969,000
Expenses
Employee compensation and benefits	13,913,000		17,488,000
Interest and dividends	81,000		19,000
Non-compensation expenses - Fixed	9,138,000		6,969,000
Non-compensation expenses - Variable	1,006,000		1,309,000
Non-compensation expenses	0		0
Reimbursement from affiliates	(1,420,000)		(1,116,000)
Total expenses	22,718,000		24,669,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0
Total other income (loss)	0		0
Income (loss) before income taxes and non-controlling interests	4,969,000		10,300,000
Income taxes expense / (benefit)	0		0
Economic income (loss)/ net income (loss) before non-controlling interests	4,969,000		10,300,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,800,000)		(300,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	3,169,000		10,000,000
Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Investment banking	17,166,000	[1]	15,630,000	[2]
Brokerage	28,017,000	[1]	24,013,000	[2]
Management fees	0	[1]	0	[2]
Incentive income	0	[1]	0	[2]
Investment Income	2,568,000	[1]	4,302,000	[2]
Interest and dividends	0	[1]	0	[2]
Reimbursement from affiliates	0	[1]	0	[2]
Other revenue	(553,000)	[1]	260,000	[2]
Total revenues	47,198,000	[1]	44,205,000	[2]
Expenses
Employee compensation and benefits	30,608,000	[1]	28,421,000	[2]
Interest and dividends	43,000	[1]	69,000	[2]
Non-compensation expenses - Fixed	14,640,000	[1]	14,274,000	[2]
Non-compensation expenses - Variable	6,345,000	[1]	5,566,000	[2]
Non-compensation expenses	0	[1]	0	[2]
Reimbursement from affiliates	0	[1]	0	[2]
Total expenses	51,636,000	[1]	48,330,000	[2]
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0	[1]	0	[2]
Total other income (loss)	0	[1]	0	[2]
Income (loss) before income taxes and non-controlling interests	(4,438,000)	[1]	(4,125,000)	[2]
Income taxes expense / (benefit)	0	[1]	0	[2]
Economic income (loss)/ net income (loss) before non-controlling interests	(4,438,000)	[1]	(4,125,000)	[2]
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	0	[1]	0	[2]
Net income (loss) attributable to Cowen Group, Inc. stockholders	(4,438,000)	[1]	(4,125,000)	[2]
Investment Income from Entity's Capital	2,300,000		4,200,000
Compensation Expense Related to Investment of Entity's Capital	800,000		1,400,000
Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking	0		0
Brokerage	0		0
Management fees	(311,000)		(393,000)
Incentive income	0		0
Investment Income	0		0
Interest and dividends	0		0
Reimbursement from affiliates	65,000		(71,000)
Other revenue	0		0
Total revenues	(159,000)		(319,000)
Expenses
Employee compensation and benefits	0		0
Interest and dividends	0		0
Non-compensation expenses - Fixed	0		0
Non-compensation expenses - Variable	0		0
Non-compensation expenses	0		0
Reimbursement from affiliates	0		0
Total expenses	434,000		375,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0
Total other income (loss)	2,293,000		2,641,000
Income (loss) before income taxes and non-controlling interests	1,700,000		1,947,000
Income taxes expense / (benefit)	0		0
Economic income (loss)/ net income (loss) before non-controlling interests	1,700,000		1,947,000
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	(1,700,000)		(1,947,000)
Net income (loss) attributable to Cowen Group, Inc. stockholders	0		0
Other Adjustments | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Investment banking	0		0
Brokerage	(1,417,000)	[3]	0
Management fees	(4,340,000)	[4]	(3,910,000)	[4]
Incentive income	(2,515,000)	[4]	(3,331,000)	[4]
Investment Income	(10,872,000)	[5]	(21,105,000)	[5]
Interest and dividends	9,321,000	[5]	5,372,000	[5]
Reimbursement from affiliates	1,420,000	[6]	1,116,000	[6]
Other revenue	918,000	[5]	483,000	[5]
Total revenues	(7,485,000)		(21,375,000)
Expenses
Employee compensation and benefits	(298,000)		774,000
Interest and dividends	6,294,000	[5]	1,636,000	[5]
Non-compensation expenses - Fixed	(23,778,000)	[5],[7]	(21,243,000)	[5],[7]
Non-compensation expenses - Variable	(7,351,000)	[5],[7]	(6,875,000)	[5],[7]
Non-compensation expenses	32,037,000	[5],[7]	27,957,000	[5],[7]
Reimbursement from affiliates	1,420,000	[6]	1,116,000	[6]
Total expenses	8,324,000		3,365,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	11,807,000	[5]	19,671,000	[5]
Total other income (loss)	14,673,000		22,996,000
Income (loss) before income taxes and non-controlling interests	(1,136,000)		(1,744,000)
Income taxes expense / (benefit)	176,000	[6]	142,000	[6]
Economic income (loss)/ net income (loss) before non-controlling interests	(1,312,000)		(1,886,000)
(Income) loss attributable to redeemable non-controlling interests in consolidated subsidiaries	5,000		6,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	(1,307,000)		(1,880,000)
Consolidated Funds | Reported Under U.S. GAAP
Revenues
Total revenues	87,000		145,000
Expenses
Total expenses	434,000		375,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	5,159,000		5,966,000
Consolidated Funds | Operating Segments | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0		0
Expenses
Total expenses	0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0
Consolidated Funds | Alternative Investment | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0		0
Expenses
Total expenses	0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0		0
Consolidated Funds | Broker-Dealer | Reported Under Economic Income/(Loss)
Revenues
Total revenues	0	[1]	0	[2]
Expenses
Total expenses	0	[1]	0	[2]
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	0	[1]	0	[2]
Consolidated Funds | Funds Consolidation | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues	87,000		145,000
Expenses
Total expenses	434,000		375,000
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	2,293,000		2,641,000
Consolidated Funds | Other Adjustments | Reconciliation from Economic Income/(Loss) to U.S. GAAP
Revenues
Total revenues	0		0
Expenses
Total expenses	0		0
Other income (loss)
Net gains (losses) on securities, derivatives and other investments	$ 2,866,000		$ 3,325,000

[1] For the three months ended March 31, 2013, the Company has reflected $2.3 million of investment income and related compensation expense of $0.8 million within the broker-dealer segment in proportion to its capital.

[2] For the three months ended March 31, 2012, the Company has reflected $4.2 million of investment income and related compensation expense of $1.4 million within the broker-dealer segment in proportion to its capital.

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

[4] 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 (2012 and 2011 only).

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

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