ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	$ (743,934)	$ (733,051)	$ (2,205,937)	$ (2,161,293)
Realized investment gains (losses): All other investments	18,150	24,152	94,708	194,663
Net impairment losses recognized in earnings	(273)	(3,308)	(10,889)	(6,892)
Income before income tax	110,850	133,079	348,967	473,582
Tax benefit (expense)	(28,308)	(39,785)	(106,743)	(152,820)
Net income	82,542	93,294	242,224	320,762
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	(38)		(396)
Income before income tax	(38)		(396)
Tax benefit (expense)	14		139
Net income	(24)		(257)
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	690	1,665	9,084	24,152
Net impairment losses recognized in earnings	(273)	(3,308)	(10,889)	(6,892)
Income before income tax	417	(1,643)	(1,805)	17,260
Tax benefit (expense)	(146)	575	631	(6,041)
Net income	$ 271	(1,068)	$ (1,174)	11,219
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Postretirement benefits liability adjustment
Reclassifications out of accumulated other comprehensive income (loss)
Amortization of net actuarial gain/(loss)		(29)		(29)
Amortization of prior service credit/(cost)		0		0
Amortization of transition asset/(obligation)		0		0
Income before income tax		(29)		(29)
Tax benefit (expense)		10		10
Net income		$ (19)		$ (19)