DERIVATIVE FINANCIAL INSTRUMENTS - Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	$ 78,059	$ (69,878)
Derivatives not designated as hedging instruments
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	78,059	(69,878)
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	82,658	(17,865)
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	(40)	3
Derivatives not designated as hedging instruments | VA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	(19,829)	(50,604)
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	(16,892)	3,448
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	(6,428)	(30,817)
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	(7,583)	(6,256)
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	12,016	(40,185)
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	(14)	(1,469)
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	(63,710)	(8,957)
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	6,490	0
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	56,292	33,632
Derivatives not designated as hedging instruments | FIA
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	6,500	(1,414)
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	(161)	297
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	(4,669)	10,700
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	11,330	(12,411)
Derivatives not designated as hedging instruments | IUL
Notional amount and fair value of the entity's derivative financial instruments
Total realized gains (losses) - derivatives	8,770	2
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	136	(799)
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	(1,250)	2,891
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	$ 9,884	$ (2,090)