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, 2017	Dec. 31, 2016
Investment [Line Items]
Amortized Cost	$ 39,651,286	$ 38,233,986
Gross Unrealized Gains	567,982	199,654
Gross Unrealized Losses	(863,181)	(1,862,975)
Fair Value	39,356,087	36,570,665
Total OTTI Recognized in OCI	(77)	(11,039)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,187,460	1,913,413
Gross Unrealized Gains	22,141	10,737
Gross Unrealized Losses	(17,347)	(25,667)
Fair Value	2,192,254	1,898,483
Total OTTI Recognized in OCI	16	(9)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,939,832	1,850,620
Gross Unrealized Gains	6,167	2,528
Gross Unrealized Losses	(24,570)	(41,678)
Fair Value	1,921,429	1,811,470
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,186,463	1,210,490
Gross Unrealized Gains	17,692	21,741
Gross Unrealized Losses	(13,488)	(20,698)
Fair Value	1,190,667	1,211,533
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,317,910	1,308,192
Gross Unrealized Gains	1,256	422
Gross Unrealized Losses	(26,333)	(40,455)
Fair Value	1,292,833	1,268,159
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	253,026	253,182
Gross Unrealized Gains	8,145	1,536
Gross Unrealized Losses	(6,835)	(14,797)
Fair Value	254,336	239,921
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	1,756,865	1,760,837
Gross Unrealized Gains	6,416	1,224
Gross Unrealized Losses	(59,272)	(105,558)
Fair Value	1,704,009	1,656,503
Total OTTI Recognized in OCI	(93)	0
Corporate securities
Investment [Line Items]
Amortized Cost	29,673,641	28,801,768
Gross Unrealized Gains	480,875	153,715
Gross Unrealized Losses	(705,147)	(1,583,918)
Fair Value	29,449,369	27,371,565
Total OTTI Recognized in OCI	0	(11,030)
Preferred stock
Investment [Line Items]
Amortized Cost	94,362	94,362
Gross Unrealized Gains	376	0
Gross Unrealized Losses	(2,898)	(8,519)
Fair Value	91,840	85,843
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	38,409,559	37,192,864
Gross Unrealized Gains	543,068	191,903
Gross Unrealized Losses	(855,890)	(1,841,290)
Fair Value	38,096,737	35,543,477
Total OTTI Recognized in OCI	(77)	(11,039)
Equity securities
Investment [Line Items]
Amortized Cost	758,804	761,340
Gross Unrealized Gains	24,914	7,751
Gross Unrealized Losses	(7,291)	(21,685)
Fair Value	776,427	747,406
Total OTTI Recognized in OCI	0	0
Short-term investments
Investment [Line Items]
Amortized Cost	482,923	279,782
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	482,923	279,782
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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