INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Total	$ 52,187,516	$ 39,455,935
Gross Unrealized Gains	195,809	712,957
Gross Unrealized Losses	(2,363,386)	(680,458)
Fair Value	50,019,939	39,488,434
Total OTTI Recognized in OCI	0	(28)
Residential mortgage-backed securities
Investment [Line Items]
Total	3,506,384	2,321,811
Gross Unrealized Gains	5,808	19,412
Gross Unrealized Losses	(105,531)	(22,730)
Fair Value	3,406,661	2,318,493
Total OTTI Recognized in OCI	0	10
Commercial mortgage-backed securities
Investment [Line Items]
Total	2,314,831	1,885,109
Gross Unrealized Gains	180	4,931
Gross Unrealized Losses	(80,633)	(29,552)
Fair Value	2,234,378	1,860,488
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Total	1,456,768	1,234,376
Gross Unrealized Gains	13,749	20,936
Gross Unrealized Losses	(16,382)	(5,763)
Fair Value	1,454,135	1,249,549
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Total	1,653,626	1,255,244
Gross Unrealized Gains	160	185
Gross Unrealized Losses	(72,807)	(32,177)
Fair Value	1,580,979	1,223,252
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Total	546,356	280,780
Gross Unrealized Gains	2,925	9,401
Gross Unrealized Losses	(21,635)	(4,948)
Fair Value	527,646	285,233
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Total	3,691,930	1,770,299
Gross Unrealized Gains	6,070	16,959
Gross Unrealized Losses	(139,927)	(45,613)
Fair Value	3,558,073	1,741,645
Total OTTI Recognized in OCI	0	(37)
Corporate securities
Investment [Line Items]
Total	38,612,538	29,446,365
Gross Unrealized Gains	166,917	618,582
Gross Unrealized Losses	(1,919,248)	(527,401)
Fair Value	36,860,207	29,537,546
Total OTTI Recognized in OCI	0	(1)
Redeemable preferred stock
Investment [Line Items]
Total	94,362	94,362
Gross Unrealized Gains	0	232
Gross Unrealized Losses	(7,223)	(3,503)
Fair Value	87,139	91,091
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Total	51,876,795	38,288,346
Gross Unrealized Gains	195,809	690,638
Gross Unrealized Losses	(2,363,386)	(671,687)
Fair Value	49,709,218	38,307,297
Total OTTI Recognized in OCI	0	(28)
Equity securities
Investment [Line Items]
Total		696,706
Gross Unrealized Gains		22,319
Gross Unrealized Losses		(8,771)
Fair Value		710,254
Total OTTI Recognized in OCI		0
Short-term investments
Investment [Line Items]
Total	310,721	470,883
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	310,721	470,883
Total OTTI Recognized in OCI	$ 0	$ 0

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in other comprehensive income (OCI).

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