FINANCIAL INSTRUMENTS Schedule of Reported Gains and Losses on Derivatives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	$ (23,223)	$ 49,639	$ (41,841)
Commodity Contract [Member] | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(9,293)	10,474	(12,061)
Credit default swaps, purchased | Corporate Debt | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	0	0	(156)
Credit default swaps, purchased | Sovereign debt – Non U.S. | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	0	0	(90)
Forwards | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	2,507	(302)	1,009
Futures | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(399)	376	(952)
Interest rate swaps | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	136	218	(706)
Options, warrants, and rights | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	(18,455)	10,261	(13,955)
Total return swaps – equities | Net investment income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	2,281	28,612	(12,590)
Weather derivative swap | Other income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives recognized in income	0	0	$ (2,340)
Derivatives not designated as hedging instruments (notional amounts)
Derivative Instruments, Gain (Loss) [Line Items]
Entered	1,284,569	2,684,389
Exited	564,959	2,518,050
Derivatives not designated as hedging instruments (notional amounts) | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Entered	2,025	141,534
Exited	41,830	100,045
Derivatives not designated as hedging instruments (notional amounts) | Forwards
Derivative Instruments, Gain (Loss) [Line Items]
Entered	34,652	6,880
Exited	1,739	3,128
Derivatives not designated as hedging instruments (notional amounts) | Futures
Derivative Instruments, Gain (Loss) [Line Items]
Entered	38,207	1,966,368
Exited	32,537	1,988,782
Derivatives not designated as hedging instruments (notional amounts) | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Entered		20,490
Exited		0
Derivatives not designated as hedging instruments (notional amounts) | Total return swaps – equities
Derivative Instruments, Gain (Loss) [Line Items]
Entered	258,874	86,688
Exited	$ 355,446	$ 59,810