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, 2017	Dec. 31, 2016
Investment [Line Items]
Amortized Cost	$ 38,262,236	$ 38,233,986
Gross Unrealized Gains	273,302	199,654
Gross Unrealized Losses	(1,569,416)	(1,862,975)
Fair Value	36,966,122	36,570,665
Total OTTI Recognized in OCI	(5,341)	(11,039)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,988,289	1,913,413
Gross Unrealized Gains	11,023	10,737
Gross Unrealized Losses	(29,120)	(25,667)
Fair Value	1,970,192	1,898,483
Total OTTI Recognized in OCI	(3)	(9)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,850,816	1,850,620
Gross Unrealized Gains	3,225	2,528
Gross Unrealized Losses	(39,378)	(41,678)
Fair Value	1,814,663	1,811,470
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,177,426	1,210,490
Gross Unrealized Gains	22,172	21,741
Gross Unrealized Losses	(17,148)	(20,698)
Fair Value	1,182,450	1,211,533
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,310,138	1,308,192
Gross Unrealized Gains	401	422
Gross Unrealized Losses	(35,744)	(40,455)
Fair Value	1,274,795	1,268,159
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	253,129	253,182
Gross Unrealized Gains	4,130	1,536
Gross Unrealized Losses	(12,058)	(14,797)
Fair Value	245,201	239,921
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	1,776,716	1,760,837
Gross Unrealized Gains	1,811	1,224
Gross Unrealized Losses	(105,803)	(105,558)
Fair Value	1,672,724	1,656,503
Total OTTI Recognized in OCI	0	0
Corporate securities
Investment [Line Items]
Amortized Cost	28,785,229	28,801,768
Gross Unrealized Gains	213,518	153,715
Gross Unrealized Losses	(1,316,535)	(1,583,918)
Fair Value	27,682,212	27,371,565
Total OTTI Recognized in OCI	(5,338)	(11,030)
Preferred stock
Investment [Line Items]
Amortized Cost	94,362	94,362
Gross Unrealized Gains	316	0
Gross Unrealized Losses	(5,404)	(8,519)
Fair Value	89,274	85,843
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	37,236,105	37,192,864
Gross Unrealized Gains	256,596	191,903
Gross Unrealized Losses	(1,561,190)	(1,841,290)
Fair Value	35,931,511	35,543,477
Total OTTI Recognized in OCI	(5,341)	(11,039)
Equity securities
Investment [Line Items]
Amortized Cost	778,213	761,340
Gross Unrealized Gains	16,706	7,751
Gross Unrealized Losses	(8,226)	(21,685)
Fair Value	786,693	747,406
Total OTTI Recognized in OCI	0	0
Short-term investments
Investment [Line Items]
Amortized Cost	247,918	279,782
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	247,918	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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