RECLASSIFICATION FROM ACCUMULATED OTHER COMPREHENSIVE INCOME INTO NET INCOME (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment (OTTI) on securities	$ (1,598)	$ (1,509)	$ (5,115)	$ (2,112)	$ (2,426)	$ (1,765)	$ (2,819)	$ (3,254)	$ (10,334)	$ (10,264)	$ (5,788)
Net income (loss)	$ 300,370	$ 239,721	$ 337,403	$ 137,158	$ 242,065	$ 131,086	$ 93,627	$ 115,938	1,014,652	582,716	281,882
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)									(10,308)	37,724	(7,308)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain (loss) on settlement of investments, net									(20,642)	27,460	(13,096)
Other-than-temporary impairment (OTTI) on securities									$ 10,334	$ 10,264	$ 5,788