FINANCIAL INSTRUMENTS Investments in Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Corporate debt – U.S.
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains	$ 0	$ 6,509
Unrealized losses	(6,874)	(5,331)
Total equity securities
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains	216,589	115,838
Unrealized losses	(49,737)	(40,706)
Exchange traded funds
Investments Sold, Not yet Purchased [Line Items]
Unrealized gains	1,972	459
Unrealized losses	0	0
Securities Sold, Not yet Purchased
Investments Sold, Not yet Purchased [Line Items]
Proceeds	(742,649)	(786,500)
Unrealized gains	27,120	37,363
Unrealized losses	(174,472)	(110,765)
Fair value	(890,001)	(859,902)
Securities Sold, Not yet Purchased | Total equity securities
Investments Sold, Not yet Purchased [Line Items]
Proceeds	(646,419)	(690,270)
Unrealized gains	27,120	30,768
Unrealized losses	(173,333)	(110,765)
Fair value	(792,632)	(770,267)
Securities Sold, Not yet Purchased | Sovereign debt – Non U.S.
Investments Sold, Not yet Purchased [Line Items]
Proceeds	(96,230)	(96,230)
Unrealized gains	0	6,595
Unrealized losses	(1,139)	0
Fair value	$ (97,369)	$ (89,635)