SHARE-BASED COMPENSATION (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Total compensation expense and related income tax benefits
Number of plans	2
RSUs
Total compensation expense and related income tax benefits
Compensation expense	$ 32,385	$ 32,772	$ 26,674
Income tax benefits	9,838	11,011	4,926
Incremental compensation expense due to modifications	255	2,210	66
Total unrecognized compensation cost	56,873
Weighted average period over which unrecognized compensation cost is expected to be recognized	1 year 8 months 26 days
Total fair value of shares vested	31,514	27,407	22,950
Stock option
Total compensation expense and related income tax benefits
Expiration term of options	10 years
Additional disclosures
Intrinsic value of options exercised	2	52	661
Intrinsic value of options outstanding and exercisable	$ 139
2008 Equity Incentive Plan
SHARE-BASED COMPENSATION
Shares of common stock available for future grants of awards	11,200,253
2008 Equity Incentive Plan and 2004 Equity Incentive Plan | RSUs
RSUs
Outstanding at the beginning of the period (in shares)	17,957,726	14,545,137	10,208,185
Granted (in shares)	8,354,723	9,250,544	8,128,286
Vested (in shares)	(6,476,243)	(5,095,883)	(3,075,790)
Cancelled (in shares)	(482,824)	(742,072)	(715,544)
Outstanding at the end of the period (in shares)	19,353,382	17,957,726	14,545,137
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 4.84	$ 5.14	$ 5.57
Granted (in dollars per share)	$ 3.55	$ 4.74	$ 5.50
Vested (in dollars per share)	$ 4.87	$ 5.38	$ 7.46
Cancelled (in dollars per share)	$ 4.84	$ 5.69	$ 5.50
Outstanding at the end of the period (in dollars per share)	$ 4.27	$ 4.84	$ 5.14
GFI Group 2002 Plan | Stock option
Options
Outstanding at the beginning of the period (in shares)	585,748	606,804	641,436
Exercised (in shares)	(10,528)	(4,212)	(30,420)
Cancelled (in shares)		(16,844)	(4,212)
Outstanding at the end of the period (in shares)	575,220	585,748	606,804
Exercisable at the end of the period (in shares)	575,220
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 3.28	$ 3.33	$ 3.31
Exercised (in dollars per share)	$ 2.97	$ 2.97	$ 2.97
Cancelled (in dollars per share)		$ 5.25	$ 2.97
Outstanding at the end of the period (in dollars per share)	$ 3.29	$ 3.28	$ 3.33
Exercisable at the end of the period (in dollars per share)	$ 3.29
Additional disclosures
Weighted Average Contractual Term, Outstanding at the end of the period	1 year 1 month 17 days
Weighted Average Contractual Term, Exercisable at the end of the period	1 year 1 month 17 days
GFInet 2000 Plan | Stock option
Options
Outstanding at the beginning of the period (in shares)	16,844	96,248	313,572
Exercised (in shares)		(23,884)	(205,960)
Cancelled (in shares)			(11,364)
Expired (in shares)		(55,520)
Outstanding at the end of the period (in shares)	16,844	16,844	96,248
Exercisable at the end of the period (in shares)	16,844
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 2.97	$ 3.95	$ 3.19
Exercised (in dollars per share)		$ 2.62	$ 2.69
Cancelled (in dollars per share)			$ 5.68
Expired (in dollars per share)		$ 4.82
Outstanding at the end of the period (in dollars per share)	$ 2.97	$ 2.97	$ 3.95
Exercisable at the end of the period (in dollars per share)	$ 2.97
Additional disclosures
Weighted Average Contractual Term, Outstanding at the end of the period	11 months 26 days
Weighted Average Contractual Term, Exercisable at the end of the period	11 months 26 days