ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications (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	$ (860,860)	$ (711,644)	$ (1,647,209)	$ (1,461,094)
Realized investment gains (losses): All other investments	(49,643)	53,535	(137,194)	76,376
Net impairment losses recognized in earnings	(5)	(2,785)	(3,650)	(7,986)
Income before income tax	60,894	133,286	82,830	219,930
Tax (expense) or benefit	(8,626)	(43,654)	(12,287)	(71,959)
Net income	52,268	89,632	70,543	147,971
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)
Income before income 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	5,483	(1,087)	8,266	8,394
Net impairment losses recognized in earnings	(5)	(2,785)	(3,650)	(7,986)
Income before income tax	5,478	(3,872)	4,616	408
Tax (expense) or benefit	(1,150)	1,355	(969)	(143)
Net income	$ 4,328	$ (2,517)	$ 3,647	$ 265