ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications (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	$ (1,171,034)			$ (950,942)			$ (3,147,641)	$ (2,598,151)
Net impairment losses recognized in earnings	(10,818)			(14)			(14,658)	(3,664)
Tax (expense) benefit	(49,417)			(16,646)			(115,355)	(28,933)
Net income	190,424	$ 154,414	$ 142,098	78,524	$ 52,268	$ 18,275	486,936	149,067
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)			21,885	6,248
Net impairment losses recognized in earnings	(10,818)			(14)			(14,658)	(3,664)
Tax (expense) benefit	(1,022)			427			(1,517)	(542)
Net income	$ 3,846			$ (1,605)			$ 5,710	$ 2,042