ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (952,353)	$ (743,934)	$ (2,600,703)	$ (2,205,937)
Realized investment gains (losses): All other investments	(19,869)	18,150	(157,070)	94,708
Net impairment losses recognized in earnings	(14)	(273)	(3,664)	(10,889)
Total before tax	136,719	110,850	327,289	348,967
Tax (expense) or benefit	(26,619)	(28,308)	(61,582)	(106,743)
Net income	110,100	82,542	265,707	242,224
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	(481)	(38)	(799)	(396)
Total before tax	(481)	(38)	(799)	(396)
Tax (expense) or benefit	101	14	168	139
Net income	(380)	(24)	(631)	(257)
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	(2,018)	690	6,304	9,084
Net impairment losses recognized in earnings	(14)	(273)	(3,664)	(10,889)
Total before tax	(2,032)	417	2,640	(1,805)
Tax (expense) or benefit	427	(146)	(554)	631
Net income	$ (1,605)	$ 271	$ 2,086	$ (1,174)