INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	$ 52,826,470	$ 39,783,880
Gross Unrealized Gains	189,175	718,229
Gross Unrealized Losses	(2,750,781)	(682,181)
Fair Value	50,264,864	39,819,928
Total OTTI Recognized in OCI	(28,827)	(2,560)
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	3,650,539	2,330,832
Gross Unrealized Gains	23,247	19,413
Gross Unrealized Losses	(62,196)	(23,033)
Fair Value	3,611,590	2,327,212
Total OTTI Recognized in OCI	(18)	41
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	2,349,274	1,914,998
Gross Unrealized Gains	3,911	5,010
Gross Unrealized Losses	(58,101)	(30,186)
Fair Value	2,295,084	1,889,822
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	1,683,432	1,255,244
Gross Unrealized Gains	1,795	185
Gross Unrealized Losses	(45,722)	(32,177)
Fair Value	1,639,505	1,223,252
Total OTTI Recognized in OCI	0	0
Other government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	545,522	282,767
Gross Unrealized Gains	4,292	9,463
Gross Unrealized Losses	(33,850)	(4,948)
Fair Value	515,964	287,282
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	38,634,888	29,606,484
Gross Unrealized Gains	112,992	623,713
Gross Unrealized Losses	(2,385,052)	(528,187)
Fair Value	36,362,828	29,702,010
Total OTTI Recognized in OCI	(29,685)	(2,564)
Redeemable preferred stock
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	52,050,113	38,489,362
Gross Unrealized Gains	189,175	695,911
Gross Unrealized Losses	(2,750,781)	(673,410)
Fair Value	49,488,507	38,511,863
Total OTTI Recognized in OCI	(28,827)	(2,560)
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost		735,569
Gross Unrealized Gains		22,318
Gross Unrealized Losses		(8,771)
Fair Value		749,116
Total OTTI Recognized in OCI		0
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	776,357	558,949
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	776,357	558,949
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 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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