INVESTMENT OPERATIONS - Amortized Cost and Fair Value of Investments (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Trading securities:
Amortized Cost	$ 52,452,334	$ 39,455,935
Gross Unrealized Gains	188,621	712,957
Gross Unrealized Losses	(2,742,547)	(680,458)
Fair Value	49,898,408	39,488,434
Total OTTI Recognized in OCI	(28,827)	(2,559)
Residential mortgage-backed securities
Trading securities:
Amortized Cost	3,641,678	2,321,811
Gross Unrealized Gains	23,248	19,412
Gross Unrealized Losses	(61,935)	(22,730)
Fair Value	3,602,991	2,318,493
Total OTTI Recognized in OCI	(18)	41
Commercial mortgage-backed securities
Trading securities:
Amortized Cost	2,319,476	1,885,109
Gross Unrealized Gains	3,911	4,931
Gross Unrealized Losses	(57,000)	(29,552)
Fair Value	2,266,387	1,860,488
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Trading securities:
Amortized Cost	1,410,059	1,234,376
Gross Unrealized Gains	17,232	20,936
Gross Unrealized Losses	(35,398)	(5,763)
Fair Value	1,391,893	1,249,549
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Trading securities:
Amortized Cost	1,658,433	1,255,244
Gross Unrealized Gains	1,794	185
Gross Unrealized Losses	(45,722)	(32,177)
Fair Value	1,614,505	1,223,252
Total OTTI Recognized in OCI	0	0
Other government-related securities
Trading securities:
Amortized Cost	543,534	280,780
Gross Unrealized Gains	4,292	9,401
Gross Unrealized Losses	(33,790)	(4,948)
Fair Value	514,036	285,233
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Trading securities:
Amortized Cost	3,682,037	1,770,299
Gross Unrealized Gains	25,706	16,959
Gross Unrealized Losses	(118,902)	(45,613)
Fair Value	3,588,841	1,741,645
Total OTTI Recognized in OCI	876	(37)
Corporate securities
Trading securities:
Amortized Cost	38,467,380	29,446,365
Gross Unrealized Gains	112,438	618,582
Gross Unrealized Losses	(2,378,240)	(527,401)
Fair Value	36,201,578	29,537,546
Total OTTI Recognized in OCI	(29,685)	(2,563)
Redeemable preferred stock
Trading securities:
Amortized Cost	94,362	94,362
Gross Unrealized Gains	0	232
Gross Unrealized Losses	(11,560)	(3,503)
Fair Value	82,802	91,091
Total OTTI Recognized in OCI	0	0
Fixed Maturities
Trading securities:
Amortized Cost	51,816,959	38,288,346
Gross Unrealized Gains	188,621	690,638
Gross Unrealized Losses	(2,742,547)	(671,687)
Fair Value	49,263,033	38,307,297
Total OTTI Recognized in OCI	(28,827)	(2,559)
Equity securities
Trading securities:
Amortized Cost		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
Trading securities:
Amortized Cost	635,375	470,883
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	635,375	470,883
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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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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