ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)
Benefits and settlement expenses, net of reinsurance ceded									$ (3,511,252)	$ (2,955,005)	$ (2,876,726)
Realized investment gains (losses): All other investments									(223,276)	121,087	90,630
Net impairment losses recognized in earnings	$ (26,060)	$ (14)	$ (5)	$ (3,645)	$ (853)	$ (273)	$ (2,785)	$ (5,201)	(29,724)	(9,112)	(17,748)
Tax (expense) or benefit	(24,728)	(16,646)	(8,626)	(3,661)	839,384	(49,016)	(43,654)	(28,305)	(53,661)	718,409	(170,073)
Net of tax	$ 44,860	$ 78,524	$ 52,268	$ 18,275	$ 917,480	$ 116,966	$ 89,632	$ 58,339	193,927	1,182,417	352,608
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									(1,431)	(694)	(60)
Tax (expense) or benefit									301	243	21
Net of tax									(1,130)	(451)	(39)
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 investment gains (losses): All other investments									9,851	10,453	32,275
Net impairment losses recognized in earnings									(29,724)	(9,112)	(17,748)
Tax (expense) or benefit									4,174	(397)	(5,085)
Net of tax									$ (15,699)	$ 944	$ 9,442