INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Amortized Cost	$ 61,618,378	$ 52,452,334
Gross Unrealized Gains	1,970,526	188,621
Gross Unrealized Losses	(615,440)	(2,742,547)
Fair Value	62,973,464	49,898,408
Total OTTI Recognized in OCI	1,954	(28,827)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	4,473,723	3,641,678
Gross Unrealized Gains	132,603	23,248
Gross Unrealized Losses	(8,882)	(61,935)
Fair Value	4,597,444	3,602,991
Total OTTI Recognized in OCI	0	(18)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,618,367	2,319,476
Gross Unrealized Gains	51,304	3,911
Gross Unrealized Losses	(3,817)	(57,000)
Fair Value	2,665,854	2,266,387
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,848,061	1,410,059
Gross Unrealized Gains	22,063	17,232
Gross Unrealized Losses	(8,526)	(35,398)
Fair Value	1,861,598	1,391,893
Total OTTI Recognized in OCI	(3)	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,304,382	1,658,433
Gross Unrealized Gains	5,179	1,794
Gross Unrealized Losses	(9,004)	(45,722)
Fair Value	1,300,557	1,614,505
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	553,779	543,534
Gross Unrealized Gains	31,425	4,292
Gross Unrealized Losses	(4,168)	(33,790)
Fair Value	581,036	514,036
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	4,652,897	3,682,037
Gross Unrealized Gains	183,920	25,706
Gross Unrealized Losses	(5,851)	(118,902)
Fair Value	4,830,966	3,588,841
Total OTTI Recognized in OCI	1,193	876
Corporate securities
Investment [Line Items]
Amortized Cost	45,103,765	38,467,380
Gross Unrealized Gains	1,542,821	112,438
Gross Unrealized Losses	(571,434)	(2,378,240)
Fair Value	46,075,152	36,201,578
Total OTTI Recognized in OCI	764	(29,685)
Redeemable preferred stocks
Investment [Line Items]
Amortized Cost	87,459	94,362
Gross Unrealized Gains	1,211	0
Gross Unrealized Losses	(3,758)	(11,560)
Fair Value	84,912	82,802
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	60,642,433	51,816,959
Gross Unrealized Gains	1,970,526	188,621
Gross Unrealized Losses	(615,440)	(2,742,547)
Fair Value	61,997,519	49,263,033
Total OTTI Recognized in OCI	1,954	(28,827)
Short-term investments
Investment [Line Items]
Amortized Cost	975,945	635,375
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	975,945	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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