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, 2011	$ 508,465		$ 1,135	$ (16,902)	$ 688,427	$ (215)	$ (163,980)	$ 104,587
Common Stock, Shares, Outstanding at Dec. 31, 2011			114,047,637
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders	(23,885)						(23,885)
Net income (loss) attributable to redeemable non-controlling interests	(72)							(72)
Defined benefit plans	423	[1]				423
Foreign currency translation	148	[1]				148
Capital contributions								500
Capital withdrawals								(20,729)
Deconsolidation of entities								(17,104)
Consolidation of entities								18,521
Restricted stock awards issued, shares			4,346,472
Common stock issued upon acquisition	0
Purchase of treasury stock, at cost	(14,826)			(14,826)
Purchase of treasury stock, at cost, shares				(5,946,217)
Treasury stock reissued during the period net of gain(loss)	0
Amortization of share based compensation	24,784
Balance at Dec. 31, 2012	495,109		1,135	(31,728)	713,211	356	(187,865)	85,703
Common Stock, Shares, Outstanding at Dec. 31, 2012			112,447,892
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders	4,647						4,647
Net income (loss) attributable to redeemable non-controlling interests	13,193							13,193
Defined benefit plans	246	[1]				246
Foreign currency translation	(10)	[1]				(10)
Capital contributions								15,181
Capital withdrawals								(28,263)
Restricted stock awards issued, shares			4,668,423
Common stock issued upon acquisition	6,240		25		6,215
Common stock issued upon acquisition (See Note 2), shares			2,491,647
Purchase of treasury stock, at cost	(16,446)			(16,446)
Purchase of treasury stock, at cost, shares				(4,606,073)
Treasury stock reissued, shares				24,744
Treasury stock reissued, cost				90
Treasury Stock Reissued at Lower than Repurchase Price							(25)
Treasury stock reissued during the period net of gain(loss)	65
Amortization of share based compensation	17,915
Balance at Dec. 31, 2013	507,766		1,160	(48,084)	737,341	592	(183,243)	85,814
Common Stock, Shares, Outstanding at Dec. 31, 2013			115,026,633
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Group, Inc. stockholders	167,216						167,216
Net income (loss) attributable to redeemable non-controlling interests	15,564							15,564
Defined benefit plans	(344)	[1]				(344)
Foreign currency translation	(231)	[1]				(231)
Capital contributions								10,441
Capital withdrawals								(24,585)
Deconsolidation of entities								(1,158)
Restricted stock awards issued, shares			4,265,339
Common stock issued upon acquisition	0
Purchase of treasury stock, at cost	(31,687)			(31,687)
Purchase of treasury stock, at cost, shares				(7,634,107)
Warrants issued	15,218
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	33,334
Stock options exercised (See Note 14)	116
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net	1,324
Treasury stock reissued, shares				0
Treasury stock reissued, cost	0
Treasury stock reissued during the period net of gain(loss)	0
Amortization of share based compensation	18,297
Balance at Dec. 31, 2014	$ 677,675		$ 1,160	$ (79,771)	$ 772,296	$ 17	$ (16,027)	$ 86,076
Common Stock, Shares, Outstanding at Dec. 31, 2014			111,691,199

[1] During the periods presented, the Company did not have material reclassifications out of other comprehensive income.