INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Amortized Cost	$ 52,559,863	$ 39,783,880
Gross Unrealized Gains	196,744	718,229
Gross Unrealized Losses	(2,370,711)	(682,181)
Fair Value	50,385,896	39,819,928
Total OTTI Recognized in OCI	0	(28)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	3,515,247	2,330,832
Gross Unrealized Gains	5,808	19,413
Gross Unrealized Losses	(105,950)	(23,033)
Fair Value	3,415,105	2,327,212
Total OTTI Recognized in OCI	0	10
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,344,653	1,914,998
Gross Unrealized Gains	181	5,010
Gross Unrealized Losses	(82,063)	(30,186)
Fair Value	2,262,771	1,889,822
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,456,768	1,234,376
Gross Unrealized Gains	13,749	20,936
Gross Unrealized Losses	(16,382)	(5,763)
Fair Value	1,454,135	1,249,549
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,653,626	1,255,244
Gross Unrealized Gains	160	185
Gross Unrealized Losses	(72,807)	(32,177)
Fair Value	1,580,979	1,223,252
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	548,343	282,767
Gross Unrealized Gains	2,926	9,463
Gross Unrealized Losses	(21,688)	(4,948)
Fair Value	529,581	287,282
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	3,691,930	1,770,299
Gross Unrealized Gains	6,070	16,959
Gross Unrealized Losses	(139,927)	(45,613)
Fair Value	3,558,073	1,741,645
Total OTTI Recognized in OCI	0	(37)
Corporate securities
Investment [Line Items]
Amortized Cost	38,778,746	29,606,484
Gross Unrealized Gains	167,850	623,713
Gross Unrealized Losses	(1,924,671)	(528,187)
Fair Value	37,021,925	29,702,010
Total OTTI Recognized in OCI	0	(1)
Redeemable preferred stock
Investment [Line Items]
Amortized Cost	94,362	94,362
Gross Unrealized Gains	0	232
Gross Unrealized Losses	(7,223)	(3,503)
Fair Value	87,139	91,091
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	52,083,675	38,489,362
Gross Unrealized Gains	196,744	695,911
Gross Unrealized Losses	(2,370,711)	(673,410)
Fair Value	49,909,708	38,511,863
Total OTTI Recognized in OCI	0	(28)
Equity securities
Investment [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
Investment [Line Items]
Amortized Cost	476,188	558,949
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	476,188	558,949
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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