Earnings Per Share (Details) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
		Dec. 05, 2016	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2016 shares	Dec. 31, 2015 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) | $			$ 9,828	$ 15,259	$ 50,502	$ (19,897)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds | $			5,162	18,478	35,412	(2,524)
Net income (loss) attributable to Cowen Group, Inc. | $			4,666	(3,219)	15,090	(17,373)
Preferred stock dividends | $			1,698	1,698	5,094	5,094
Net income (loss) attributable to Cowen Group, Inc. common stockholders | $			$ 2,968	$ (4,917)	$ 9,996	$ (22,467)
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	[1]		31,271,000	26,993,000	29,004,000	26,818,000
Weighted average shares used in diluted computation, shares	[1]		32,246,000	26,993,000	30,011,000	26,818,000
Earnings (loss) per share:
Earnings Per Share, Basic (in dollars per share) | $ / shares	[1]		$ 0.09	$ (0.18)	$ 0.34	$ (0.84)
Earnings Per Share, Diluted (in dollars per share) | $ / shares	[1]		$ 0.09	$ (0.18)	$ 0.33	$ (0.84)
Performance based restricted stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares			0	0	0	0
Stock Appreciation Rights
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares			20,000	0	15,000	0
Restricted Stock
Shares for basic and diluted calculations:
Shares attributable to share-based payment awards, shares			955,000	0	992,000	0
Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares			31,087,458	26,828,538	31,087,458	26,828,538	26,731,289	26,401,163
Common stock, restricted shares, shares			191,962		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.