ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)
Benefits and settlement expenses, net of reinsurance ceded									$ (4,900,537)	$ (4,256,062)	$ (3,511,252)
Realized gains (losses): investments									(31,127)	177,086	(173,903)
Tax (expense) benefit	$ (68)	$ (12,850)	$ (40,736)	$ (17,508)	$ (15,109)	$ (49,417)	$ (31,309)	$ (34,629)	(71,162)	(130,464)	(53,661)
Net income	$ 43,530	$ 59,762	$ 165,105	$ 73,965	$ 66,048	$ 190,424	$ 154,414	$ 142,098	342,362	552,984	193,927
Gains and losses on derivative instruments | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)
Benefits and settlement expenses, net of reinsurance ceded									(3,101)	(2,278)	(1,431)
Tax (expense) benefit									651	479	301
Net income									(2,450)	(1,799)	(1,130)
Unrealized gains and losses on available-for-sale securities | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)
Realized gains (losses): investments									45,324	47,711	9,851
Net impairment losses recognized in earnings									(125,470)	(34,453)	(29,724)
Tax (expense) benefit									16,831	(2,784)	4,174
Net income									$ (63,315)	$ 10,474	$ (15,699)