FINANCIAL INSTRUMENTS Schedule of Investments Categorized by the Level of Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instruments	$ 7,180	$ 22,473
Listed equity securities	1,203,672	844,001
Commodities	121,502	137,296
Private and unlisted equity securities	6,108	6,109
Total investment assets	1,338,462	1,009,879
Alternative Investment, Fair Value Disclosure	24,522	12,658
Investments	1,362,984	1,022,537
Financial contracts receivable, at fair value	12,893	76,381
Listed equity securities, sold not yet purchased	(812,652)	(770,267)
Debt instruments, sold not yet purchased	100,145	89,635
Total securities sold, not yet purchased	(912,797)	(859,902)
Financial contracts payable	(22,222)	(2,237)
Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instruments	0	0
Listed equity securities	1,181,150	823,421
Commodities	121,502	137,296
Private and unlisted equity securities	0	0
Total investment assets	1,302,652	960,717
Financial contracts receivable, at fair value	22	20
Listed equity securities, sold not yet purchased	(812,652)	(770,267)
Debt instruments, sold not yet purchased	0	0
Total securities sold, not yet purchased	(812,652)	(770,267)
Financial contracts payable	0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instruments	6,300	21,819
Listed equity securities	22,522	20,580
Commodities	0	0
Private and unlisted equity securities	0	0
Total investment assets	28,822	42,399
Financial contracts receivable, at fair value	12,871	76,361
Listed equity securities, sold not yet purchased	0	0
Debt instruments, sold not yet purchased	100,145	89,635
Total securities sold, not yet purchased	(100,145)	(89,635)
Financial contracts payable	(22,222)	(2,237)
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instruments	880	654
Listed equity securities	0	0
Commodities	0	0
Private and unlisted equity securities	6,108	6,109
Total investment assets	6,988	6,763
Financial contracts receivable, at fair value	0	0
Listed equity securities, sold not yet purchased	0	0
Debt instruments, sold not yet purchased	0	0
Total securities sold, not yet purchased	0	0
Financial contracts payable	$ 0	$ 0