ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (1,132,174)		$ (1,003,453)		$ (2,476,336)	$ (1,976,607)
Tax (expense) benefit	(40,736)		(31,309)		(58,244)	(65,938)
Net credit losses recognized in operations	(30,288)				(82,081)
Net income	165,105	$ 73,965	154,414	$ 142,098	239,070	296,512
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	(1,467)				(2,671)
Tax (expense) benefit	308				561
Net income	(1,159)				(2,110)
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			(336)			(614)
Tax (expense) benefit			70			128
Net income			(266)			(486)
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)
Tax (expense) benefit	5,847		(76)		8,498	(495)
Realized gains (losses) - investments	2,447		1,062		41,617	6,199
Net credit losses recognized in operations	(30,288)				(82,081)
Net credit losses recognized in operations			(698)			(3,840)
Net income	$ (21,994)		$ 288		$ (31,966)	$ 1,864