ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded(1)	$ (972,766)	$ (786,802)
Net impairment losses recognized in earnings	(3,142)	(3,645)
Tax (expense) benefit	(36,631)	(17,686)
Net income	138,284	74,112
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(1)	(278)
Tax (expense) benefit	58
Net income	(220)
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(1)		(113)
Tax (expense) benefit		24
Net income		(89)
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	5,117	2,783
Net impairment losses recognized in earnings	(3,142)	(3,645)
Tax (expense) benefit	(415)	181
Net income	$ 1,560	$ (681)