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	$ 38,540,325	$ 39,455,935
Gross Unrealized Gains	300,378	712,957
Gross Unrealized Losses	(1,439,751)	(680,458)
Fair Value	37,400,952	39,488,434
Total OTTI Recognized in OCI	14	(28)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,421,346	2,321,811
Gross Unrealized Gains	11,722	19,412
Gross Unrealized Losses	(47,411)	(22,730)
Fair Value	2,385,657	2,318,493
Total OTTI Recognized in OCI	14	10
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,869,751	1,885,109
Gross Unrealized Gains	574	4,931
Gross Unrealized Losses	(57,586)	(29,552)
Fair Value	1,812,739	1,860,488
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	280,677	280,780
Gross Unrealized Gains	6,407	9,401
Gross Unrealized Losses	(11,626)	(4,948)
Fair Value	275,458	285,233
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	29,324,542	29,446,365
Gross Unrealized Gains	259,633	618,582
Gross Unrealized Losses	(1,185,858)	(527,401)
Fair Value	28,398,317	29,537,546
Total OTTI Recognized in OCI	0	(1)
Redeemable 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	38,374,844	38,288,346
Gross Unrealized Gains	300,378	690,638
Gross Unrealized Losses	(1,439,751)	(671,687)
Fair Value	37,235,471	38,307,297
Total OTTI Recognized in OCI	14	(28)
Equity securities
Investment [Line Items]
Amortized Cost		696,706
Gross Unrealized Gains		22,319
Gross Unrealized Losses		(8,771)
Fair Value		710,254
Total OTTI Recognized in OCI		0
Short-term investments
Investment [Line Items]
Amortized Cost	165,481	470,883
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	165,481	470,883
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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