INVESTMENT OPERATIONS - Amortized Cost and Fair Value of Investments (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Trading securities:
Amortized Cost	$ 61,970,744	$ 52,452,334
Gross Unrealized Gains	3,100,992	188,621
Gross Unrealized Losses	(325,660)	(2,742,547)
Fair Value	64,746,076	49,898,408
Total OTTI Recognized in OCI	(33,351)	(28,827)
Residential mortgage-backed securities
Trading securities:
Amortized Cost	5,812,170	3,641,678
Gross Unrealized Gains	125,493	23,248
Gross Unrealized Losses	(6,322)	(61,935)
Fair Value	5,931,341	3,602,991
Total OTTI Recognized in OCI	0	(18)
Commercial mortgage-backed securities
Trading securities:
Amortized Cost	2,588,575	2,319,476
Gross Unrealized Gains	54,385	3,911
Gross Unrealized Losses	(3,292)	(57,000)
Fair Value	2,639,668	2,266,387
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Trading securities:
Amortized Cost	1,764,120	1,410,059
Gross Unrealized Gains	32,041	17,232
Gross Unrealized Losses	(14,926)	(35,398)
Fair Value	1,781,235	1,391,893
Total OTTI Recognized in OCI	(7)	0
U.S. government-related securities
Trading securities:
Amortized Cost	1,032,048	1,658,433
Gross Unrealized Gains	5,664	1,794
Gross Unrealized Losses	(5,316)	(45,722)
Fair Value	1,032,396	1,614,505
Total OTTI Recognized in OCI	0	0
Other government-related securities
Trading securities:
Amortized Cost	548,136	543,534
Gross Unrealized Gains	51,024	4,292
Gross Unrealized Losses	(1,991)	(33,790)
Fair Value	597,169	514,036
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Trading securities:
Amortized Cost	4,415,008	3,682,037
Gross Unrealized Gains	225,072	25,706
Gross Unrealized Losses	(1,230)	(118,902)
Fair Value	4,638,850	3,588,841
Total OTTI Recognized in OCI	1,236	876
Corporate securities
Trading securities:
Amortized Cost	44,493,799	38,467,380
Gross Unrealized Gains	2,603,636	112,438
Gross Unrealized Losses	(288,334)	(2,378,240)
Fair Value	46,809,101	36,201,578
Total OTTI Recognized in OCI	(34,580)	(29,685)
Redeemable preferred stocks
Trading securities:
Amortized Cost	87,237	94,362
Gross Unrealized Gains	3,677	0
Gross Unrealized Losses	(4,249)	(11,560)
Fair Value	86,665	82,802
Total OTTI Recognized in OCI	0	0
Fixed Maturities
Trading securities:
Amortized Cost	60,741,093	51,816,959
Gross Unrealized Gains	3,100,992	188,621
Gross Unrealized Losses	(325,660)	(2,742,547)
Fair Value	63,516,425	49,263,033
Total OTTI Recognized in OCI	(33,351)	(28,827)
Short-term investments
Trading securities:
Amortized Cost	1,229,651	635,375
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,229,651	635,375
Total OTTI Recognized in OCI	$ 0	$ 0

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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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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