INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Amortized Cost	$ 51,798,849	$ 39,783,880
Gross Unrealized Gains	244,410	718,229
Gross Unrealized Losses	(2,127,349)	(682,181)
Fair Value	49,915,910	39,819,928
Total OTTI Recognized in OCI	0	(28)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	3,297,665	2,330,832
Gross Unrealized Gains	12,486	19,413
Gross Unrealized Losses	(66,171)	(23,033)
Fair Value	3,243,980	2,327,212
Total OTTI Recognized in OCI	0	10
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,298,945	1,914,998
Gross Unrealized Gains	1,344	5,010
Gross Unrealized Losses	(72,887)	(30,186)
Fair Value	2,227,402	1,889,822
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,408,987	1,234,376
Gross Unrealized Gains	25,100	20,936
Gross Unrealized Losses	(10,526)	(5,763)
Fair Value	1,423,561	1,249,549
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,549,033	1,255,244
Gross Unrealized Gains	295	185
Gross Unrealized Losses	(57,292)	(32,177)
Fair Value	1,492,036	1,223,252
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	417,232	282,767
Gross Unrealized Gains	3,269	9,463
Gross Unrealized Losses	(18,771)	(4,948)
Fair Value	401,730	287,282
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	3,699,964	1,770,299
Gross Unrealized Gains	23,107	16,959
Gross Unrealized Losses	(89,268)	(45,613)
Fair Value	3,633,803	1,741,645
Total OTTI Recognized in OCI	0	(37)
Corporate securities
Investment [Line Items]
Amortized Cost	38,372,075	29,606,484
Gross Unrealized Gains	178,729	623,713
Gross Unrealized Losses	(1,807,568)	(528,187)
Fair Value	36,743,236	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	80	232
Gross Unrealized Losses	(4,866)	(3,503)
Fair Value	89,576	91,091
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	51,138,263	38,489,362
Gross Unrealized Gains	244,410	695,911
Gross Unrealized Losses	(2,127,349)	(673,410)
Fair Value	49,255,324	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	660,586	558,949
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	660,586	558,949
Total OTTI Recognized in OCI	$ 0	$ 0

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount before tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss.

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