DERIVATIVE FINANCIAL INSTRUMENTS - Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	$ (148,049)	$ 12,265	$ (267,720)	$ 90,324
Derivatives not designated as hedging instruments
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(148,049)	12,265	(267,720)	90,324
Derivatives not designated as hedging instruments | Embedded derivative - Modco reinsurance treaties
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(70,679)	45,183	(155,677)	127,841
Derivatives not designated as hedging instruments | Other derivatives
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(1,455)	33	(1,389)	(7)
Derivatives not designated as hedging instruments | VA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(56,123)	(36,115)	(86,650)	(55,944)
Derivatives not designated as hedging instruments | VA | Interest rate futures
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(11,280)	1,552	(17,302)	(15,340)
Derivatives not designated as hedging instruments | VA | Equity futures
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	2,559	(10,864)	32,297	(17,292)
Derivatives not designated as hedging instruments | VA | Currency futures
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(397)	12,063	1,847	4,480
Derivatives not designated as hedging instruments | VA | Equity options
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(21,702)	(38,216)	(93,397)	(26,200)
Derivatives not designated as hedging instruments | VA | Interest rate swaptions
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	0	0	0	(14)
Derivatives not designated as hedging instruments | VA | Interest rate swaps
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	117,934	(26,149)	192,795	(89,859)
Derivatives not designated as hedging instruments | VA | Total return swaps
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(8,545)	(10,055)	(48,572)	(3,565)
Derivatives not designated as hedging instruments | VA | Embedded derivative - GLWB
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(134,692)	35,554	(154,318)	91,846
Derivatives not designated as hedging instruments | FIA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(11,197)	2,304	(8,390)	8,804
Derivatives not designated as hedging instruments | FIA | Equity futures
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	431	(167)	2	(328)
Derivatives not designated as hedging instruments | FIA | Equity options
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	13,191	7,398	55,241	2,729
Derivatives not designated as hedging instruments | FIA | Embedded derivative - FIA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(24,819)	(4,927)	(63,633)	6,403
Derivatives not designated as hedging instruments | IUL
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(8,595)	860	(15,614)	9,630
Derivatives not designated as hedging instruments | IUL | Equity futures
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	85	0	256	136
Derivatives not designated as hedging instruments | IUL | Equity options
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	2,606	2,086	8,786	836
Derivatives not designated as hedging instruments | IUL | Embedded derivative - IUL
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	$ (11,286)	$ (1,226)	$ (24,656)	$ 8,658