ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 7,127,199	$ 5,471,521	$ 4,581,224
Cumulative effect adjustments		0
Ending Balance	5,767,734	7,127,199	5,471,521	$ 4,581,224
Unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	25,874	(656,322)	(1,247,065)
Other comprehensive income (loss) before reclassifications	(1,420,499)	707,298	602,211
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	(20,751)	391	(2,008)
Amounts reclassified from accumulated other comprehensive income (loss)	15,651	642	(9,460)
Cumulative effect adjustments	(10,552)	(26,135)
Ending Balance	(1,410,277)	25,874	(656,322)	(1,247,065)
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	613,400	(6,300)	424,100	623,000
Accumulated Gain and Loss on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	747	727	0
Other comprehensive income (loss) before reclassifications	(1,884)	(563)	688
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	1,130	451	39
Cumulative effect adjustments	0	132
Ending Balance	(7)	747	727	0
Minimum Pension Benefits Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(13,925)	1,072	5,931
Other comprehensive income (loss) before reclassifications	(3,546)	(15,726)	(5,659)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	1,989	501	800
Cumulative effect adjustments	0	228
Ending Balance	(15,482)	(13,925)	1,072	5,931
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	12,696	(654,523)	(1,241,134)
Other comprehensive income (loss) before reclassifications	(1,425,929)	691,009	597,240
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	(20,751)	391	(2,008)
Amounts reclassified from accumulated other comprehensive income (loss)	18,770	1,594	(8,621)
Cumulative effect adjustments	(10,552)	(25,775)
Ending Balance	$ (1,425,766)	$ 12,696	$ (654,523)	$ (1,241,134)

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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- Definition

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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