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, 2019	Dec. 31, 2018
Investment [Line Items]
Amortized Cost	$ 61,861,481	$ 52,452,334
Gross Unrealized Gains	3,362,167	188,621
Gross Unrealized Losses	(351,317)	(2,742,547)
Fair Value	64,872,331	49,898,408
Total OTTI Recognized in OCI	(23,373)	(28,827)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	5,151,583	3,641,678
Gross Unrealized Gains	194,100	23,248
Gross Unrealized Losses	(5,020)	(61,935)
Fair Value	5,340,663	3,602,991
Total OTTI Recognized in OCI	0	(18)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,627,310	2,319,476
Gross Unrealized Gains	88,275	3,911
Gross Unrealized Losses	(1,119)	(57,000)
Fair Value	2,714,466	2,266,387
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,878,663	1,410,059
Gross Unrealized Gains	32,830	17,232
Gross Unrealized Losses	(13,017)	(35,398)
Fair Value	1,898,476	1,391,893
Total OTTI Recognized in OCI	(3)	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,154,940	1,658,433
Gross Unrealized Gains	9,836	1,794
Gross Unrealized Losses	(2,569)	(45,722)
Fair Value	1,162,207	1,614,505
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	552,445	543,534
Gross Unrealized Gains	52,421	4,292
Gross Unrealized Losses	(1,630)	(33,790)
Fair Value	603,236	514,036
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	4,503,629	3,682,037
Gross Unrealized Gains	331,625	25,706
Gross Unrealized Losses	(597)	(118,902)
Fair Value	4,834,657	3,588,841
Total OTTI Recognized in OCI	1,274	876
Corporate securities
Investment [Line Items]
Amortized Cost	44,960,104	38,467,380
Gross Unrealized Gains	2,648,863	112,438
Gross Unrealized Losses	(323,362)	(2,378,240)
Fair Value	47,285,605	36,201,578
Total OTTI Recognized in OCI	(24,644)	(29,685)
Redeemable preferred stocks
Investment [Line Items]
Amortized Cost	87,348	94,362
Gross Unrealized Gains	4,217	0
Gross Unrealized Losses	(4,003)	(11,560)
Fair Value	87,562	82,802
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	60,916,022	51,816,959
Gross Unrealized Gains	3,362,167	188,621
Gross Unrealized Losses	(351,317)	(2,742,547)
Fair Value	63,926,872	49,263,033
Total OTTI Recognized in OCI	(23,373)	(28,827)
Short-term investments
Investment [Line Items]
Amortized Cost	945,459	635,375
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	945,459	635,375
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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