ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (742,789)	$ (732,339)	$ (2,203,883)	$ (2,158,007)
Realized investment gains (losses): All other investments	18,150	24,152	94,526	194,644
Net impairment losses recognized in earnings	(273)	(3,308)	(8,259)	(6,892)
Income before income tax	165,982	94,267	385,912	589,826
Tax (expense) or benefit	(49,016)	(20,965)	(120,975)	(185,114)
Net income	116,966	73,302	264,937	404,712
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	(38)		(396)
Income before income tax	(38)		(396)
Tax (expense) or benefit	14		139
Net income	(24)		(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	690	1,665	9,084	24,133
Net impairment losses recognized in earnings	(273)	(3,308)	(8,259)	(6,892)
Income before income tax	417	(1,643)	825	17,241
Tax (expense) or benefit	(146)	575	(289)	(6,035)
Net income	$ 271	$ (1,068)	$ 536	$ 11,206