SEGMENT AND GEOGRAPHIC INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
SEGMENT AND GEOGRAPHIC INFORMATION
Number of operating segments	5
Number of broad product categories	4
SEGMENT AND GEOGRAPHIC INFORMATION
Total revenues	$ 924,587	$ 1,015,473	$ 862,602
Revenues, net of interest and transaction-based expenses	787,045	880,771	795,044
Income (loss) before income taxes	(1,257)	82	31,803
Americas Brokerage
SEGMENT AND GEOGRAPHIC INFORMATION
Total revenues	276,350	313,483	294,910
Revenues, net of interest and transaction-based expenses	262,560	299,590	278,553
Income (loss) before income taxes	69,648	81,308	66,399
EMEA Brokerage
SEGMENT AND GEOGRAPHIC INFORMATION
Total revenues	338,504	386,012	379,033
Revenues, net of interest and transaction-based expenses	328,722	374,802	367,494
Income (loss) before income taxes	83,334	88,122	115,037
Asia Brokerage
SEGMENT AND GEOGRAPHIC INFORMATION
Total revenues	71,927	82,903	74,945
Revenues, net of interest and transaction-based expenses	71,813	82,839	74,875
Income (loss) before income taxes	15,075	15,533	15,814
Clearing and Backed Trading
SEGMENT AND GEOGRAPHIC INFORMATION
Total revenues	159,877	164,882	53,129
Revenues, net of interest and transaction-based expenses	44,597	54,104	12,140
Income (loss) before income taxes	6,688	9,196	(3,041)
All other
SEGMENT AND GEOGRAPHIC INFORMATION
Total revenues	77,929	68,193	60,585
Revenues, net of interest and transaction-based expenses	79,353	69,436	61,982
Income (loss) before income taxes	$ (176,002)	$ (194,077)	$ (162,406)