FAIR VALUE OF FINANCIAL INSTRUMENTS (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Receivables from brokers, dealers and clearing organizations	$ 252,696	$ 251,794
Other assets: Financial instruments owned	25,250	6,864
Other assets: Other:
Convertible note receivable, available-for-sale		5,362
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	1,481	976
Other liabilities: Contingent consideration			15,558
Variation margin on net long derivative contracts related to exchange traded futures included within receivables from brokers, dealers and clearing organizations	49
Variation margin on net short derivative contracts related to exchange traded futures included within receivables from brokers, dealers and clearing organizations	14,986
Variation margin on long and short derivative contracts related to exchange traded futures included within receivables from brokers, dealers and clearing organizations		1,125
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Other assets: Financial instruments owned	24,234	2,367
Other assets: Other:
Equity security, available-for-sale	3,356	2,901
Total	27,590	5,768
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	420	129
Total	420	129
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Assets
Receivables from brokers, dealers and clearing organizations		500
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Assets
Other assets: Financial instruments owned	1,223	480
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	205	129
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative contracts
Assets
Other assets: Financial instruments owned	23,011	1,887
Netting	(17,961)	(1,210)
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	215
Netting	(17,961)	(1,210)
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign exchange derivative contracts
Assets
Other assets: Financial instruments owned		16
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	14	7
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income derivative contracts
Assets
Other assets: Financial instruments owned	394	1,628
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	595	384
Fair value measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivative contracts
Assets
Other assets: Financial instruments owned	40,578	1,453
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	17,567	819
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Other assets: Financial instruments owned	988	2,560
Other assets: Other:
Total	988	2,560
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	1,061	847
Total	1,061	847
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Other assets: Financial instruments owned	199	208
Liabilities
Other liabilities: Financial instruments sold, not yet purchased		22
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Derivative contracts
Assets
Other assets: Financial instruments owned	789	2,352
Netting	(182,955)	(183,581)
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	1,061	825
Netting	(182,912)	(183,529)
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Foreign exchange derivative contracts
Assets
Other assets: Financial instruments owned	182,343	185,933
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	182,573	184,354
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Commodity derivative contracts
Assets
Other assets: Financial instruments owned	1,401
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	1,400
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Other assets: Financial instruments owned	28	1,937
Other assets: Other:
Convertible note receivable, available-for-sale		5,362
Total	28	7,299
Liabilities
Other liabilities: Future purchase commitment	3,209	12,562
Other liabilities: Contingent consideration	518	1,119
Total	3,727	13,681
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Derivative contracts
Assets
Other assets: Financial instruments owned	28	1,937
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Equity derivative contracts
Assets
Other assets: Financial instruments owned	28	1,937
Other assets: Other:
Total	28
Fair value measured on a recurring basis | Total
Assets
Other assets: Financial instruments owned	25,250	6,864
Other assets: Other:
Equity security, available-for-sale	3,356	2,901
Convertible note receivable, available-for-sale		5,362
Total	28,606	15,627
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	1,481	976
Other liabilities: Future purchase commitment	3,209	12,562
Other liabilities: Contingent consideration	518	1,119
Total	5,208	14,657
Fair value measured on a recurring basis | Total | U.S. Treasury securities
Assets
Receivables from brokers, dealers and clearing organizations		500
Fair value measured on a recurring basis | Total | Equity securities
Assets
Other assets: Financial instruments owned	1,422	688
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	205	151
Fair value measured on a recurring basis | Total | Derivative contracts
Assets
Other assets: Financial instruments owned	23,828	6,176
Netting	(200,916)	(184,791)
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	1,276	825
Netting	(200,873)	(184,739)
Fair value measured on a recurring basis | Total | Foreign exchange derivative contracts
Assets
Other assets: Financial instruments owned	182,343	185,949
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	182,587	184,361
Fair value measured on a recurring basis | Total | Fixed income derivative contracts
Assets
Other assets: Financial instruments owned	394	1,628
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	595	384
Fair value measured on a recurring basis | Total | Equity derivative contracts
Assets
Other assets: Financial instruments owned	40,606	3,390
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	17,567	819
Fair value measured on a recurring basis | Total | Commodity derivative contracts
Assets
Other assets: Financial instruments owned	1,401
Liabilities
Other liabilities: Financial instruments sold, not yet purchased	$ 1,400