ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications (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	$ (950,942)	$ (742,789)	$ (2,598,151)	$ (2,203,883)
Realized investment gains (losses): All other investments	(20,156)	18,150	(157,350)	94,526
Net impairment losses recognized in earnings	(14)	(273)	(3,664)	(8,259)
Income before income tax	95,170	165,982	178,000	385,912
Tax (expense) or benefit	(16,646)	(49,016)	(28,933)	(120,975)
Net income	78,524	116,966	149,067	264,937
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)
Income before income 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	6,248	9,084
Net impairment losses recognized in earnings	(14)	(273)	(3,664)	(8,259)
Income before income tax	(2,032)	417	2,584	825
Tax (expense) or benefit	427	(146)	(542)	(289)
Net income	$ (1,605)	$ 271	$ 2,042	$ 536