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, 2017	Dec. 31, 2016
Investment [Line Items]
Amortized Cost	$ 38,337,534	$ 38,233,986
Gross Unrealized Gains	459,878	199,654
Gross Unrealized Losses	(1,012,264)	(1,862,975)
Fair Value	37,785,148	36,570,665
Total OTTI Recognized in OCI	(1,368)	(11,039)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,006,404	1,913,413
Gross Unrealized Gains	14,311	10,737
Gross Unrealized Losses	(19,578)	(25,667)
Fair Value	2,001,137	1,898,483
Total OTTI Recognized in OCI	8	(9)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,852,497	1,850,620
Gross Unrealized Gains	6,039	2,528
Gross Unrealized Losses	(26,407)	(41,678)
Fair Value	1,832,129	1,811,470
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,179,284	1,210,490
Gross Unrealized Gains	19,712	21,741
Gross Unrealized Losses	(16,993)	(20,698)
Fair Value	1,182,003	1,211,533
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,321,287	1,308,192
Gross Unrealized Gains	1,351	422
Gross Unrealized Losses	(26,530)	(40,455)
Fair Value	1,296,108	1,268,159
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	253,115	253,182
Gross Unrealized Gains	6,550	1,536
Gross Unrealized Losses	(8,161)	(14,797)
Fair Value	251,504	239,921
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	1,755,780	1,760,837
Gross Unrealized Gains	5,154	1,224
Gross Unrealized Losses	(71,613)	(105,558)
Fair Value	1,689,321	1,656,503
Total OTTI Recognized in OCI	0	0
Corporate securities
Investment [Line Items]
Amortized Cost	28,790,475	28,801,768
Gross Unrealized Gains	379,523	153,715
Gross Unrealized Losses	(833,444)	(1,583,918)
Fair Value	28,336,554	27,371,565
Total OTTI Recognized in OCI	(1,376)	(11,030)
Preferred stock
Investment [Line Items]
Amortized Cost	94,362	94,362
Gross Unrealized Gains	1,791	0
Gross Unrealized Losses	(3,085)	(8,519)
Fair Value	93,068	85,843
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	37,253,204	37,192,864
Gross Unrealized Gains	434,431	191,903
Gross Unrealized Losses	(1,005,811)	(1,841,290)
Fair Value	36,681,824	35,543,477
Total OTTI Recognized in OCI	(1,368)	(11,039)
Equity securities
Investment [Line Items]
Amortized Cost	763,136	761,340
Gross Unrealized Gains	25,447	7,751
Gross Unrealized Losses	(6,453)	(21,685)
Fair Value	782,130	747,406
Total OTTI Recognized in OCI	0	0
Short-term investments
Investment [Line Items]
Amortized Cost	321,194	279,782
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	321,194	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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