ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (786,802)	$ (749,642)
Realized investment gains (losses): All other investments	(87,599)	22,841
Net impairment losses recognized in earnings	(3,645)	(7,831)
Total before tax	91,798	112,349
Tax (expense) or benefit	(17,686)	(36,935)
Net income	74,112	75,414
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	(113)	(205)
Total before tax	(113)	(205)
Tax (expense) or benefit	24	72
Net income	(89)	(133)
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	2,783	9,481
Net impairment losses recognized in earnings	(3,645)	(7,831)
Total before tax	(862)	1,650
Tax (expense) or benefit	181	(578)
Net income	$ (681)	$ 1,072