FINANCIAL INSTRUMENTS Schedule of Investments in Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equities – listed
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains	$ 208,350	$ 115,838
Unrealized losses	(19,104)	(40,706)
Exchange traded funds
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains		459
Unrealized losses		0
Corporate debt – U.S.
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains	0	6,509
Unrealized losses	(7,186)	(5,331)
Sovereign debt – Non U.S.
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains	0
Unrealized losses	(25)
Securities Sold, Not yet Purchased
Investments Sold, Not yet Purchased [Line Items]
Proceeds	(739,379)	(786,500)
Unrealized gains	17,541	37,363
Unrealized losses	(190,959)	(110,765)
Fair value	(912,797)	(859,902)
Securities Sold, Not yet Purchased | Equities – listed
Investments Sold, Not yet Purchased [Line Items]
Proceeds	(643,148)	(690,270)
Unrealized gains	17,541	30,768
Unrealized losses	(187,045)	(110,765)
Fair value	(812,652)	(770,267)
Securities Sold, Not yet Purchased | Sovereign debt – Non U.S.
Investments Sold, Not yet Purchased [Line Items]
Proceeds	(96,231)	(96,230)
Unrealized gains	0	6,595
Unrealized losses	(3,914)	0
Fair value	$ (100,145)	$ (89,635)