ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in AOCI by Component (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 8,324,285
Total other comprehensive income (loss)	(571,748)	$ 160,965
Ending Balance	7,682,218		$ 8,324,285
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	340,200		(6,300)	$ 424,100
Unrealized Gains and Losses on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	23,069	(655,767)	(655,767)
Other comprehensive income (loss) before reclassifications	(573,016)		699,097
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	681		7,153
Amounts reclassified from accumulated other comprehensive income (loss)	11		(944)
Total other comprehensive income (loss)	(572,324)		705,306
Cumulative effect adjustments	(10,552)		(26,470)
Ending Balance	(559,807)		23,069	(655,767)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	747	727	727
Other comprehensive income (loss) before reclassifications	487		(563)
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)	89		451
Total other comprehensive income (loss)	576		(112)
Cumulative effect adjustments	0		132
Ending Balance	1,323		747	727
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	23,816	$ (655,040)	(655,040)
Other comprehensive income (loss) before reclassifications	(572,529)		698,534
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	681		7,153
Amounts reclassified from accumulated other comprehensive income (loss)	100		(493)
Total other comprehensive income (loss)	(571,748)		705,194
Cumulative effect adjustments	(10,552)		(26,338)
Ending Balance	$ (558,484)		$ 23,816	$ (655,040)

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

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- Definition

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 comprehensive income (loss) attributable to parent entity.

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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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