ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (1,003,987)		$ (861,548)		$ (1,976,753)	$ (1,648,350)
Net impairment losses recognized in earnings	(698)		(5)		(3,840)	(3,650)
Tax (expense) benefit	(21,963)		(17,277)		(58,594)	(34,963)
Net income	108,111	$ 138,284	81,495	$ 74,112	246,395	155,607
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Gains and losses on derivative instruments
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	(336)				(614)
Tax (expense) benefit	70				128
Net income	(266)				(486)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Gains and losses on derivative instruments
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded			(205)			(318)
Tax (expense) benefit			43			67
Net income			(162)			(251)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Unrealized gains and losses on available-for-sale securities
Reclassifications out of accumulated other comprehensive income (loss)
Realized investment gains (losses): All other investments	1,098		5,539		6,215	8,322
Net impairment losses recognized in earnings	(698)		(5)		(3,840)	(3,650)
Tax (expense) benefit	(84)		(1,162)		(499)	(981)
Net income	$ 316		$ 4,372		$ 1,876	$ 3,691