DERIVATIVE FINANCIAL INSTRUMENTS - Realized Investment Gains (Losses), Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	$ 60,988	$ (305,828)	$ (40,288)
Derivatives not designated as hedging instruments
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	60,988	(305,828)	(40,288)
Derivatives not designated as hedging instruments | Embedded derivative - Modco reinsurance treaties
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	166,757	(103,009)	390
Derivatives not designated as hedging instruments | Other derivatives
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	14	50	(24)
Derivatives not designated as hedging instruments | VA
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(105,610)	(186,863)	(49,692)
Derivatives not designated as hedging instruments | VA | Interest rate futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(25,473)	26,015	(3,450)
Derivatives not designated as hedging instruments | VA | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(88,208)	(91,776)	(106,431)
Derivatives not designated as hedging instruments | VA | Currency futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	10,275	(23,176)	33,836
Derivatives not designated as hedging instruments | VA | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	38,083	(94,791)	(60,962)
Derivatives not designated as hedging instruments | VA | Interest rate swaptions
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(14)	(2,490)	(1,161)
Derivatives not designated as hedging instruments | VA | Interest rate swaps
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(45,185)	27,981	20,420
Derivatives not designated as hedging instruments | VA | Total return swaps
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	77,225	(32,240)	0
Derivatives not designated as hedging instruments | VA | Embedded derivative - GLWB
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(72,313)	3,614	68,056
Derivatives not designated as hedging instruments | FIA
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(3,158)	(10,651)	(4,097)
Derivatives not designated as hedging instruments | FIA | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	330	642	4,248
Derivatives not designated as hedging instruments | FIA | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(38,885)	44,585	8,149
Derivatives not designated as hedging instruments | FIA | Embedded derivative—FIA
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	35,397	(55,878)	(16,494)
Derivatives not designated as hedging instruments | IUL
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	2,985	(5,355)	13,135
Derivatives not designated as hedging instruments | IUL | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	261	(818)	129
Derivatives not designated as hedging instruments | IUL | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(6,338)	9,580	3,477
Derivatives not designated as hedging instruments | IUL | Embedded derivative—IUL
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	$ 9,062	$ (14,117)	$ 9,529