ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 5,767,734	$ 7,127,199	$ 7,127,199
Ending Balance	6,997,027	6,392,342	5,767,734
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,425,766)	12,696	12,696
Other comprehensive income (loss) before reclassifications	1,134,365	(1,425,929)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	8,792	(20,751)
Amounts reclassified from accumulated other comprehensive income (loss)	(1,340)	18,770
Cumulative effect adjustments	0	(10,552)
Ending Balance	(283,949)	(574,300)	(1,425,766)
Unrealized Gains and Losses on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,410,277)	25,874	25,874
Other comprehensive income (loss) before reclassifications	1,136,331	(1,420,499)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	8,792	(20,751)
Amounts reclassified from accumulated other comprehensive income (loss)	(1,560)	15,651
Cumulative effect adjustments	0	(10,552)
Ending Balance	(266,714)		(1,410,277)
Offset of net unrealized losses in AOCI due to impact those net unrealized losses would have on certain of the Company's insurance assets and liabilities had the net unrealized losses been recognized in net income	199,100		613,400
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(7)	747	747
Other comprehensive income (loss) before reclassifications		(1,884)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings		0
Amounts reclassified from accumulated other comprehensive income (loss)		1,130
Cumulative effect adjustments		0
Ending Balance			(7)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications	(1,966)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0
Amounts reclassified from accumulated other comprehensive income (loss)	220
Cumulative effect adjustments	0
Ending Balance	(1,753)
Minimum Pension Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(15,482)	(13,925)	(13,925)
Other comprehensive income (loss) before reclassifications	0	(3,546)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	1,989
Cumulative effect adjustments	0	$ 0
Ending Balance	$ (15,482)		$ (15,482)

Accumulated Other Comprehensive Income (Loss), Adjustment of Net Unrealized Losses Reported

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Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other than temporary impairment loss (OTTI) on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss and attributable to parent entity.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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