ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (861,548)	$ (712,361)	$ (1,648,350)	$ (1,462,003)
Realized investment gains (losses): All other investments	(49,602)	53,717	(137,201)	76,558
Net impairment losses recognized in earnings	(5)	(2,785)	(3,650)	(10,616)
Total before tax	98,772	125,768	190,570	238,117
Tax (expense) or benefit	(17,277)	(41,500)	(34,963)	(78,435)
Net income	81,495	84,268	155,607	159,682
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	(205)	(153)	(318)	(358)
Total before tax	(205)	(153)	(318)	(358)
Tax (expense) or benefit	43	53	67	125
Net income	(162)	(100)	(251)	(233)
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	5,539	(1,087)	8,322	8,394
Net impairment losses recognized in earnings	(5)	(2,785)	(3,650)	(10,616)
Total before tax	5,534	(3,872)	4,672	(2,222)
Tax (expense) or benefit	(1,162)	1,355	(981)	777
Net income	$ 4,372	$ (2,517)	$ 3,691	$ (1,445)