ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (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									$ (3,515,869)	$ (2,957,270)	$ (2,880,435)
Realized investment gains (losses): All other investments									(223,649)	121,428	90,659
Net impairment losses recognized in earnings	$ (26,060)	$ (14)	$ (5)	$ (3,645)	$ (853)	$ (273)	$ (2,785)	$ (7,831)	(29,724)	(11,742)	(17,748)
Tax (expense) or benefit	(19,075)	(26,619)	(17,277)	(17,686)	778,218	(28,308)	(41,500)	(36,935)	(80,657)	671,475	(200,968)
Net income	$ 36,654	$ 110,100	$ 81,495	$ 74,112	$ 864,308	$ 82,542	$ 84,268	$ 75,414	302,361	1,106,532	393,029
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 income									(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									9,912	10,611	32,302
Net impairment losses recognized in earnings									(29,724)	(11,742)	(17,748)
Tax (expense) or benefit									4,161	489	(5,094)
Net income									(15,651)	(642)	9,460
Pension benefits liability adjustment | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Amortization of net actuarial gain/(loss)									(2,518)	(634)	(1,231)
Tax (expense) or benefit									529	133	431
Net income									$ (1,989)	$ (501)	$ (800)