Earnings Per Share (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Warrants and Rights	Jun. 30, 2013 Warrants and Rights	Jun. 30, 2014 Warrants and Rights	Jun. 30, 2013 Warrants and Rights	Jun. 30, 2014 Stock Options	Jun. 30, 2013 Stock Options	Jun. 30, 2014 Stock Options	Jun. 30, 2013 Stock Options	Jun. 30, 2014 Stock Appreciation Rights (SARs)	Jun. 30, 2013 Stock Appreciation Rights (SARs)	Jun. 30, 2014 Stock Appreciation Rights (SARs)	Jun. 30, 2013 Stock Appreciation Rights (SARs)	Jun. 30, 2014 Restricted Stock	Jun. 30, 2013 Restricted Stock	Jun. 30, 2014 Restricted Stock	Jun. 30, 2013 Restricted Stock	Jun. 30, 2014 Common Stock Class A	Dec. 31, 2013 Common Stock Class A
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Common stock, shares outstanding, shares																					114,966,803	115,026,633
Common stock, restricted shares, shares																					424,479	482,522
Computation of earnings per share:
Net income (loss)	$ 13,598,000	$ 3,348,000	$ 27,625,000	$ 4,267,000
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and funds	5,216,000	2,255,000	9,403,000	5,750,000
Net income (loss) attributable to Cowen Group, Inc. stockholders	$ 8,382,000	$ 1,093,000	$ 18,222,000	$ (1,483,000)
Shares for basic and diluted calculations:
Weighted average shares used in basic computation, shares	115,569,000	117,235,000	115,626,000	115,471,000
Shares attributable to share-based payment awards, shares					0	0	0	0	0	0	0	0	58,000	0	56,000	0	4,572,000	3,666,000	4,953,000	0
Weighted average shares used in diluted computation, shares	120,199,000	120,901,000	120,635,000	115,471,000
Earnings (loss) per share:
Earnings Per Share, Basic	$ 0.07	$ 0.01	$ 0.16	$ (0.01)
Earnings Per Share, Diluted	$ 0.07	$ 0.01	$ 0.15	$ (0.01)