INVESTMENT OPERATIONS - Schedule of Available-for-Sale Credit Losses on Fixed Maturities Held by the Company for Which a Portion of Other-than-Temporary Impairments were Recognized in Other Comprehensive Income (Loss) (Details) - Fixed maturities - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 3,268	$ 12,685
Additions for newly impaired securities	0	0
Additions for previously impaired securities	0	0
Reductions for previously impaired securities due to a change in expected cash flows	(1,033)	(12,685)
Reductions for previously impaired securities that were sold in the current period	0	0
Ending balance	$ 2,235	$ 0