DERIVATIVE FINANCIAL INSTRUMENTS - Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	$ (27,465)	$ (82,341)	$ 62,859	$ (260,407)
Derivatives not designated as hedging instruments
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(27,465)	(82,341)	62,859	(260,407)
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	10,811	(19,746)	138,652	(90,314)
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	(51)	43	(58)	41
Derivatives not designated as hedging instruments | VA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(37,343)	(57,076)	(93,287)	(154,056)
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	2,111	549	(13,229)	16,746
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	(5,733)	(25,959)	(23,025)	(75,389)
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	3,410	(6,092)	7,890	(22,366)
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	(21,206)	(23,307)	(47,406)	(76,376)
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	(292)	(14)	(2,423)
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	(41,288)	5,342	(131,147)	31,331
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	(32,343)	(8,057)	(35,908)	(9,675)
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	57,706	740	149,552	(15,904)
Derivatives not designated as hedging instruments | FIA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	3,726	(7,298)	12,530	(10,679)
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	(388)	66	(716)	161
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	18,474	11,242	21,203	29,511
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	(14,360)	(18,606)	(7,957)	(40,351)
Derivatives not designated as hedging instruments | IUL
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(4,608)	1,736	5,022	(5,399)
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	(1)	58	135	(878)
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	4,928	1,975	5,764	6,437
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	$ (9,535)	$ (297)	$ (877)	$ (10,958)