INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Amortized Cost	$ 52,656,572	$ 52,826,470
Gross Unrealized Gains	768,862	189,175
Gross Unrealized Losses	(1,358,532)	(2,750,781)
Fair Value	52,066,902	50,264,864
Total OTTI Recognized in OCI	(17,698)	(28,827)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	3,880,305	3,650,539
Gross Unrealized Gains	57,389	23,247
Gross Unrealized Losses	(29,406)	(62,196)
Fair Value	3,908,288	3,611,590
Total OTTI Recognized in OCI	0	(18)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,363,988	2,349,274
Gross Unrealized Gains	17,199	3,911
Gross Unrealized Losses	(27,843)	(58,101)
Fair Value	2,353,344	2,295,084
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,359,813	1,410,059
Gross Unrealized Gains	18,421	17,232
Gross Unrealized Losses	(15,623)	(35,398)
Fair Value	1,362,611	1,391,893
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,439,136	1,683,432
Gross Unrealized Gains	2,382	1,795
Gross Unrealized Losses	(30,412)	(45,722)
Fair Value	1,411,106	1,639,505
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	523,701	545,522
Gross Unrealized Gains	12,343	4,292
Gross Unrealized Losses	(11,883)	(33,850)
Fair Value	524,161	515,964
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	3,632,880	3,682,037
Gross Unrealized Gains	84,040	25,706
Gross Unrealized Losses	(25,256)	(118,902)
Fair Value	3,691,664	3,588,841
Total OTTI Recognized in OCI	1,021	876
Corporate securities
Investment [Line Items]
Amortized Cost	38,586,159	38,634,888
Gross Unrealized Gains	576,720	112,992
Gross Unrealized Losses	(1,213,985)	(2,385,052)
Fair Value	37,948,894	36,362,828
Total OTTI Recognized in OCI	(18,719)	(29,685)
Redeemable preferred stocks
Investment [Line Items]
Amortized Cost	87,579	94,362
Gross Unrealized Gains	368	0
Gross Unrealized Losses	(4,124)	(11,560)
Fair Value	83,823	82,802
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	51,873,561	52,050,113
Gross Unrealized Gains	768,862	189,175
Gross Unrealized Losses	(1,358,532)	(2,750,781)
Fair Value	51,283,891	49,488,507
Total OTTI Recognized in OCI	(17,698)	(28,827)
Short-term investments
Investment [Line Items]
Amortized Cost	783,011	776,357
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	783,011	776,357
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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