FINANCIAL INSTRUMENTS Schedule of Investments Categorized by Level of Fair Value Hierarchy (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets:		$ 1,323,077	$ 1,009,879
Debt instruments		1,737	22,473
Listed equity securities		1,195,864	844,001
Commodities		119,349	137,296
Private and unlisted equity securities		6,127	6,109
Private equity funds measured at net asset value (1)	[1]	13,689	12,658
Total investments		1,336,766	1,022,537
Financial contracts receivable		20,644	76,381
Listed equity securities, sold not yet purchased		(792,632)	(770,267)
Debt instruments, sold not yet purchased		(97,369)	(89,635)
Total securities sold, not yet purchased		(890,001)	(859,902)
Financial contracts payable		(10,409)	(2,237)
Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets:		1,297,428	960,717
Debt instruments		0	0
Listed equity securities		1,178,079	823,421
Commodities		119,349	137,296
Private and unlisted equity securities		0	0
Financial contracts receivable		16	20
Listed equity securities, sold not yet purchased		(792,632)	(770,267)
Debt instruments, sold not yet purchased		0	0
Total securities sold, not yet purchased		(792,632)	(770,267)
Financial contracts payable		(34)	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets:		18,688	42,399
Debt instruments		903	21,819
Listed equity securities		17,785	20,580
Commodities		0	0
Private and unlisted equity securities		0	0
Financial contracts receivable		20,628	76,361
Listed equity securities, sold not yet purchased		0	0
Debt instruments, sold not yet purchased		(97,369)	(89,635)
Total securities sold, not yet purchased		(97,369)	(89,635)
Financial contracts payable		(10,375)	(2,237)
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets:		6,961	6,763
Debt instruments		834	654
Listed equity securities		0	0
Commodities		0	0
Private and unlisted equity securities		6,127	6,109
Financial contracts receivable		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

[1] Investments measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. The fair value amounts are presented in the above table to facilitate reconciliation to the condensed consolidated balance sheets.