DERIVATIVE FINANCIAL INSTRUMENTS - Realized Investment Gains (Losses), Derivative Instruments (Details) - USD ($) $ in Thousands	1 Months Ended	11 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		$ 29,997	$ (305,828)	$ (40,288)
Derivatives not designated as hedging instruments
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		29,997	(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		166,092	(103,009)	390
Derivatives not designated as hedging instruments | Other derivatives
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		91	50	(24)
Derivatives not designated as hedging instruments | Annuity account
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(135,299)	(186,863)	(49,692)
Derivatives not designated as hedging instruments | Annuity account | Interest rate futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(14,818)	26,015	(3,450)
Derivatives not designated as hedging instruments | Annuity account | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(5,033)	(91,776)	(106,431)
Derivatives not designated as hedging instruments | Annuity account | Currency futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		7,169	(23,176)	33,836
Derivatives not designated as hedging instruments | Annuity account | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(27,733)	(94,791)	(60,962)
Derivatives not designated as hedging instruments | Annuity account | Interest rate swaptions
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(13,354)	(2,490)	(1,161)
Derivatives not designated as hedging instruments | Annuity account | Interest rate swaps
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(85,942)	27,981	20,420
Derivatives not designated as hedging instruments | Annuity account | Total Return Swap
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		0	(32,240)	0
Derivatives not designated as hedging instruments | Annuity account | Embedded derivative - GLWB
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		4,412	3,614	68,056
Derivatives not designated as hedging instruments | FIA
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		123	(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		(355)	642	4,248
Derivatives not designated as hedging instruments | FIA | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		1,211	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		(738)	(55,878)	(16,494)
Derivatives not designated as hedging instruments | FIA | Volatility futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		5	0	0
Derivatives not designated as hedging instruments | IUL
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(1,010)	(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		144	(818)	129
Derivatives not designated as hedging instruments | IUL | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross		(540)	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		$ (614)	$ (14,117)	$ 9,529
Predecessor
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	$ (123,274)
Predecessor | Derivatives not designated as hedging instruments
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(123,274)
Predecessor | Derivatives not designated as hedging instruments | Embedded derivative - Modco reinsurance treaties
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(68,026)
Predecessor | Derivatives not designated as hedging instruments | Other derivatives
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(37)
Predecessor | Derivatives not designated as hedging instruments | Annuity account
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(53,581)
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Interest rate futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	1,413
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	9,221
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Currency futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	7,778
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	3,047
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Interest rate swaptions
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	9,268
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Interest rate swaps
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	122,710
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Total Return Swap
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	0
Predecessor | Derivatives not designated as hedging instruments | Annuity account | Embedded derivative - GLWB
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(207,018)
Predecessor | Derivatives not designated as hedging instruments | FIA
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(1,032)
Predecessor | Derivatives not designated as hedging instruments | FIA | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(184)
Predecessor | Derivatives not designated as hedging instruments | FIA | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(2,617)
Predecessor | Derivatives not designated as hedging instruments | FIA | Embedded derivative—FIA
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	1,769
Predecessor | Derivatives not designated as hedging instruments | FIA | Volatility futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	0
Predecessor | Derivatives not designated as hedging instruments | IUL
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(598)
Predecessor | Derivatives not designated as hedging instruments | IUL | Equity futures
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	3
Predecessor | Derivatives not designated as hedging instruments | IUL | Equity options
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	(115)
Predecessor | Derivatives not designated as hedging instruments | IUL | Embedded derivative—IUL
Realized investment gains (losses) - derivative instruments
Realized investment gains (losses) - derivatives, gross	$ (486)