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, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (1,172,297)			$ (952,353)			$ (3,149,050)	$ (2,600,703)
Net impairment losses recognized in earnings	(10,818)			(14)			(14,658)	(3,664)
Tax (expense) benefit	(27,217)			(26,619)			(85,811)	(61,582)
Net income	100,626	$ 108,111	$ 138,284	110,100	$ 81,495	$ 74,112	347,021	265,707
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	(746)						(1,360)
Tax (expense) benefit	157						285
Net income	(589)						(1,075)
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)				(799)
Tax (expense) benefit				101				168
Net income				(380)				(631)
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	15,686			(2,018)			21,901	6,304
Net impairment losses recognized in earnings	(10,818)			(14)			(14,658)	(3,664)
Tax (expense) benefit	(1,022)			427			(1,521)	(554)
Net income	$ 3,846			$ (1,605)			$ 5,722	$ 2,086