Earnings Per Share (Details) $ / shares in Units, $ in Thousands		3 Months Ended												12 Months Ended
	Dec. 05, 2016	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	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, 2017 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares		Dec. 31, 2015 USD ($) $ / shares shares		Dec. 31, 2014 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) | $														$ (37,091)		$ (12,395)		$ 58,975
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds | $		$ (11,621)	$ 5,162	$ 21,145	$ 9,105	$ 9,406		$ 18,478		$ (16,705)		$ (4,297)		23,791		6,882		15,246
Net income (loss) attributable to Cowen Inc. | $		(75,972)	4,666	7,440	2,984	(1,904)		(3,219)		(10,456)		(3,698)		(60,882)		(19,277)		43,729
Preferred stock dividends | $		1,698	1,698	1,698	1,698	1,698		1,698		1,698		1,698		6,792		6,792		4,075
Net income (loss) attributable to Cowen Group, Inc. common stockholders | $		$ (77,670)	$ 2,968	$ 5,742	$ 1,286	$ (3,602)		$ (4,917)		$ (12,154)		$ (5,396)		$ (67,674)		$ (26,069)		$ 39,654
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares		30,934,000	31,271,000	28,634,000	27,061,000	26,973,000	[1]	26,993,000	[1]	26,867,000	[1]	26,591,000	[1]	29,492,000	[2]	26,857,000	[2]	27,522,000	[2]
Weighted average shares used in diluted computation, shares		30,934,000	32,246,000	29,474,000	28,401,000	26,973,000	[1]	26,993,000	[1]	26,867,000	[1]	26,591,000	[1]	29,492,000	[2]	26,857,000	[2]	29,043,000	[2]
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share) | $ / shares		$ (2.51)	$ 0.09	$ 0.20	$ 0.05	$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ (2.29)	[2]	$ (0.97)	[2]	$ 1.44	[2]
Earnings Per Share, Diluted (in dollars per share) | $ / shares		$ (2.51)	$ 0.09	$ 0.19	$ 0.05	$ (0.13)	[1]	$ (0.18)	[1]	$ (0.45)	[1]	$ (0.20)	[1]	$ (2.29)	[2]	$ (0.97)	[2]	$ 1.37	[2]
Stock Options
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares														0		0		3,000
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares														0		0		65,000
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares														0		0		35,000
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares														0		0		1,418,000
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares		29,632,020				26,731,289								29,632,020		26,731,289		26,401,163		27,922,799.000
Common stock, restricted shares, shares		191,962				162,176								191,962		162,176

[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.