Condensed Consolidated Statements of Changes in Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Redeemable Non-controlling Interest
Balance at Dec. 31, 2012	$ 495,109	$ 1,135	$ (31,728)	$ 713,211	$ 356	$ (187,865)	$ 85,703
Balance, shares at Dec. 31, 2012		112,447,892
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders	(2,576)					(2,576)
Net income (loss) attributable to redeemable non-controlling interests	(3,495)						3,495
Defined benefit plans	(17)				(17)
Foreign currency translation	(4)				(4)
Capital contributions							801
Capital withdrawals							(8,968)
Restricted stock awards issued, shares		689,594
Common stock issued upon acquisition (See Note 2)	6,297	25		6,272
Common stock issued upon acquisition (See Note 2), shares		2,514,468
Purchase of treasury stock, at cost	(412)		(412)
Purchase of treasury stock, at cost, Shares		(155)
Amortization of share based compensation	4,752			4,752
Balance at Mar. 31, 2013	$ 503,149	$ 1,160	$ (32,140)	$ 724,235	$ 335	$ (190,441)	$ 81,031
Balance, shares at Mar. 31, 2013		115,651,799