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, 2018	Dec. 31, 2017
Investment [Line Items]
Amortized Cost	$ 38,949,464	$ 39,783,880
Gross Unrealized Gains	302,253	718,229
Gross Unrealized Losses	(1,444,019)	(682,181)
Fair Value	37,807,698	39,819,928
Total OTTI Recognized in OCI	14	(28)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,430,278	2,330,832
Gross Unrealized Gains	11,723	19,413
Gross Unrealized Losses	(47,631)	(23,033)
Fair Value	2,394,370	2,327,212
Total OTTI Recognized in OCI	14	10
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,899,618	1,914,998
Gross Unrealized Gains	583	5,010
Gross Unrealized Losses	(58,695)	(30,186)
Fair Value	1,841,506	1,889,822
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,238,929	1,234,376
Gross Unrealized Gains	15,376	20,936
Gross Unrealized Losses	(9,653)	(5,763)
Fair Value	1,244,652	1,249,549
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,377,633	1,255,244
Gross Unrealized Gains	168	185
Gross Unrealized Losses	(50,001)	(32,177)
Fair Value	1,327,800	1,223,252
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	282,664	282,767
Gross Unrealized Gains	6,421	9,463
Gross Unrealized Losses	(11,626)	(4,948)
Fair Value	277,459	287,282
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	1,767,604	1,770,299
Gross Unrealized Gains	6,162	16,959
Gross Unrealized Losses	(73,487)	(45,613)
Fair Value	1,700,279	1,741,645
Total OTTI Recognized in OCI	0	(37)
Corporate securities
Investment [Line Items]
Amortized Cost	29,487,086	29,606,484
Gross Unrealized Gains	261,484	623,713
Gross Unrealized Losses	(1,188,797)	(528,187)
Fair Value	28,559,773	29,702,010
Total OTTI Recognized in OCI	0	(1)
Preferred stock
Investment [Line Items]
Amortized Cost	94,362	94,362
Gross Unrealized Gains	336	232
Gross Unrealized Losses	(4,129)	(3,503)
Fair Value	90,569	91,091
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	38,578,174	38,489,362
Gross Unrealized Gains	302,253	695,911
Gross Unrealized Losses	(1,444,019)	(673,410)
Fair Value	37,436,408	38,511,863
Total OTTI Recognized in OCI	14	(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	371,290	558,949
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	371,290	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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