Earnings Per Share (Details) $ / shares in Units, $ in Thousands			3 Months Ended																12 Months Ended
		Dec. 05, 2016	Dec. 31, 2016 USD ($) $ / shares shares		Sep. 30, 2016 USD ($) $ / shares shares		Jun. 30, 2016 USD ($) $ / shares shares		Mar. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2015 USD ($) $ / shares shares		Sep. 30, 2015 USD ($) $ / shares shares		Jun. 30, 2015 USD ($) $ / shares shares		Mar. 31, 2015 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2015 USD ($) $ / shares shares		Dec. 31, 2014 USD ($) $ / shares shares		Dec. 31, 2013 shares
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio		0.25
Computation of earnings per share:
Net income (loss) | $																			$ (12,395)		$ 58,975		$ 182,780
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds | $			$ 9,406		$ 18,478		$ (16,705)		$ (4,297)		$ 4,266		$ 4,344		$ 3,916		$ 2,720		6,882		15,246		15,564
Net income (loss) attributable to Cowen Group, Inc. | $			(1,904)		(3,219)		(10,456)		(3,698)		30,661		(10,346)		6,716		16,698		(19,277)		43,729		167,216
Preferred stock dividends | $			1,698		1,698		1,698		1,698		1,717		1,603		755		0		6,792		4,075		0
Net income (loss) attributable to Cowen Group, Inc. common stockholders | $			$ (3,602)		$ (4,917)		$ (12,154)		$ (5,396)		$ 28,944		$ (11,949)		$ 5,961		$ 16,698		$ (26,069)		$ 39,654		$ 167,216
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares			26,973,000	[1]	26,993,000	[1]	26,867,000	[1]	26,591,000	[1]	26,809,000	[1]	27,297,000	[1]	27,978,000	[1]	28,013,000	[1]	26,857,000	[2]	27,522,000	[2]	28,731,000	[2],[3]
Weighted average shares used in diluted computation, shares			26,973,000	[1]	26,993,000	[1]	26,867,000	[1]	26,591,000	[1]	28,182,000	[1]	27,297,000	[1]	29,556,000	[1]	29,647,000	[1]	26,857,000	[2],[3]	29,043,000	[2],[3]	29,871,000	[2],[3]
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share) | $ / shares			$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ 1.08	[1]	$ (0.44)	[1]	$ 0.21	[1]	$ 0.60	[1]	$ (0.97)	[2],[3]	$ 1.44	[2],[3]	$ 5.82	[2],[3]
Earnings Per Share, Diluted (in dollars per share) | $ / shares			$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ 1.03	[1]	$ (0.44)	[1]	$ 0.20	[1]	$ 0.56	[1]	$ (0.97)	[2],[3]	$ 1.37	[2],[3]	$ 5.60	[2],[3]
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	[2]																		0		3,000		0
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	[2]																		0		65,000		0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	[2]																		0		35,000		15,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares	[2]																		0		1,418,000		1,125,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares			26,731,289								26,401,163								26,731,289		26,401,163		27,922,799		28,756,658
Common stock, restricted shares, shares			162,176								124,392								162,176		124,392

[1] Share and per share amounts have been retroactively updated to reflect the one-for-four reverse stock split effective as of December 5, 2016.

[2] Share and per share amounts have been retroactively updated to reflect the one-for-four reverse stock split effective as of December 5, 2016.

[3] Share and per share amounts have been retroactively updated to reflect the one-for-four reverse stock split effective as of December 5, 2016.