DERIVATIVE FINANCIAL INSTRUMENTS - Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	$ (102,403)	$ (27,465)	$ (370,123)	$ 62,859
Derivatives not designated as hedging instruments
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(102,403)	(27,465)	(370,123)	62,859
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	(44,027)	10,811	(199,704)	138,652
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	(1,622)	(51)	(3,011)	(58)
Derivatives not designated as hedging instruments | VA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(65,104)	(37,343)	(151,754)	(93,287)
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	727	2,111	(16,575)	(13,229)
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	5,220	(5,733)	37,517	(23,025)
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	9,181	3,410	11,028	7,890
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	(3,957)	(21,206)	(97,354)	(47,406)
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	149,766	(41,288)	342,561	(131,147)
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	(1,950)	(32,343)	(50,522)	(35,908)
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	(224,091)	57,706	(378,409)	149,552
Derivatives not designated as hedging instruments | FIA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	1,412	3,726	(6,978)	12,530
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	962	(388)	964	(716)
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	4,785	18,474	60,026	21,203
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	(4,335)	(14,360)	(67,968)	(7,957)
Derivatives not designated as hedging instruments | IUL
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	6,938	(4,608)	(8,676)	5,022
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	91	(1)	347	135
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	586	4,928	9,372	5,764
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	$ 6,261	$ (9,535)	$ (18,395)	$ (877)