INVESTMENT OPERATIONS - Schedule of Investments' Gross Unrealized Losses and Fair Value of the Company's Investments that are Not Deemed to be Other-than-Temporarily Impaired (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value
Less Than 12 Months	$ 13,232,367	$ 18,267,283
12 Months or More	9,970,850	10,957,881
Total	23,203,217	29,225,164
Unrealized Loss
Less Than 12 Months	(307,536)	(666,780)
12 Months or More	(704,728)	(1,196,195)
Total	(1,012,264)	(1,862,975)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,025,391	1,060,569
12 Months or More	78,327	170,826
Total	1,103,718	1,231,395
Unrealized Loss
Less Than 12 Months	(17,400)	(21,550)
12 Months or More	(2,178)	(4,117)
Total	(19,578)	(25,667)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	1,311,451	1,452,146
12 Months or More	92,718	100,475
Total	1,404,169	1,552,621
Unrealized Loss
Less Than 12 Months	(22,749)	(37,665)
12 Months or More	(3,658)	(4,013)
Total	(26,407)	(41,678)
Other asset-backed securities
Fair Value
Less Than 12 Months	215,084	323,706
12 Months or More	169,548	176,792
Total	384,632	500,498
Unrealized Loss
Less Than 12 Months	(5,730)	(9,291)
12 Months or More	(11,263)	(11,407)
Total	(16,993)	(20,698)
U.S. government-related securities
Fair Value
Less Than 12 Months	1,182,816	1,237,942
12 Months or More	2	3
Total	1,182,818	1,237,945
Unrealized Loss
Less Than 12 Months	(26,530)	(40,454)
12 Months or More	0	(1)
Total	(26,530)	(40,455)
Other government-related securities
Fair Value
Less Than 12 Months	43,859	98,412
12 Months or More	83,449	79,393
Total	127,308	177,805
Unrealized Loss
Less Than 12 Months	(467)	(2,907)
12 Months or More	(7,694)	(11,890)
Total	(8,161)	(14,797)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	893,426	1,062,368
12 Months or More	564,522	548,254
Total	1,457,948	1,610,622
Unrealized Loss
Less Than 12 Months	(35,282)	(63,809)
12 Months or More	(36,331)	(41,749)
Total	(71,613)	(105,558)
Corporate securities
Fair Value
Less Than 12 Months	8,408,650	12,553,514
12 Months or More	8,907,968	9,793,579
Total	17,316,618	22,347,093
Unrealized Loss
Less Than 12 Months	(196,514)	(469,189)
12 Months or More	(636,930)	(1,114,729)
Total	(833,444)	(1,583,918)
Preferred stock
Fair Value
Less Than 12 Months	33,561	66,781
12 Months or More	18,934	19,062
Total	52,495	85,843
Unrealized Loss
Less Than 12 Months	(1,081)	(6,642)
12 Months or More	(2,004)	(1,877)
Total	(3,085)	(8,519)
Equity securities
Fair Value
Less Than 12 Months	118,129	411,845
12 Months or More	55,382	69,497
Total	173,511	481,342
Unrealized Loss
Less Than 12 Months	(1,783)	(15,273)
12 Months or More	(4,670)	(6,412)
Total	$ (6,453)	$ (21,685)