ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								11 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded										$ (2,539,943)	$ (2,957,270)	$ (2,880,435)
Realized investment gains (losses): All other investments										(166,886)	121,428	90,659
Net impairment losses recognized in earnings		$ (853)	$ (273)	$ (2,785)	$ (7,831)	$ (10,856)	$ (3,308)	$ (967)	$ (2,617)	(26,993)	(11,742)	(17,748)
Income before income tax	$ 1,182	86,090	110,850	125,768	112,349	120,415	133,079	168,661	171,842	399,842	435,057	593,997
Tax (expense) or benefit		778,218	(28,308)	(41,500)	(36,935)	(48,148)	(39,785)	(56,541)	(56,494)	(131,543)	671,475	(200,968)
Net income		$ 864,308	$ 82,542	$ 84,268	$ 75,414	$ 72,267	$ 93,294	$ 112,120	$ 115,348	268,299	1,106,532	393,029
Accumulated Gain and Loss Derivatives | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded										(131)	(694)	(60)
Income before income tax										(131)	(694)	(60)
Tax (expense) or benefit										45	243	21
Net income										(86)	(451)	(39)
Unrealized gains and losses on available-for-sale securities | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Realized investment gains (losses): All other investments										281	10,611	32,302
Net impairment losses recognized in earnings										(26,992)	(11,742)	(17,748)
Income before income tax										(26,711)	(1,131)	14,554
Tax (expense) or benefit										9,349	489	(5,094)
Net income										$ (17,362)	(642)	9,460
Pension benefits liability adjustment
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax											(634)	(1,231)
Reclassification from AOCI, tax											133	431
Reclassification from AOCI, net of tax											(501)	(800)
Amortization of net actuarial gain/(loss)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax											(634)	(1,231)
Amortization of prior service credit/(cost)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax											0	0
Amortization of transition asset/(obligation)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax											$ 0	$ 0
Predecessor
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	(267,287)
Realized investment gains (losses): All other investments	81,153
Net impairment losses recognized in earnings	(481)
Income before income tax	1,182
Tax (expense) or benefit	327
Net income	1,509
Predecessor | Accumulated Gain and Loss Derivatives | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Benefits and settlement expenses, net of reinsurance ceded	(36)
Income before income tax	(36)
Tax (expense) or benefit	13
Net income	(23)
Predecessor | Unrealized gains and losses on available-for-sale securities | Amount Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassifications out of accumulated other comprehensive income (loss)
Realized investment gains (losses): All other investments	6,891
Net impairment losses recognized in earnings	(481)
Income before income tax	6,410
Tax (expense) or benefit	(2,244)
Net income	4,166
Predecessor | Pension benefits liability adjustment
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax	(772)
Reclassification from AOCI, tax	270
Reclassification from AOCI, net of tax	(502)
Predecessor | Amortization of net actuarial gain/(loss)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax	(808)
Predecessor | Amortization of prior service credit/(cost)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax	31
Predecessor | Amortization of transition asset/(obligation)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI, before tax	$ 5