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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (712,361)	$ (713,697)	$ (1,462,003)	$ (1,428,242)
Realized investment gains (losses): All other investments	53,717	88,783	76,558	170,511
Net impairment losses recognized in earnings	(2,785)	(967)	(10,616)	(3,584)
Income before income tax	125,768	168,661	238,117	340,503
Tax benefit (expense)	(41,500)	(56,541)	(78,435)	(113,035)
Net income	84,268	112,120	159,682	227,468
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	(153)		(358)
Income before income tax	(153)		(358)
Tax benefit (expense)	53		125
Net income	(100)		(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	(1,087)	16,932	8,394	22,487
Net impairment losses recognized in earnings	(2,785)	(967)	(10,616)	(3,584)
Income before income tax	(3,872)	15,965	(2,222)	18,903
Tax benefit (expense)	1,355	(5,588)	777	(6,616)
Net income	$ (2,517)	$ 10,377	$ (1,445)	$ 12,287