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, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 25,500,625	$ 7,026,657
12 Months or More	13,195,814	14,598,554
Total	38,696,439	21,625,211
Unrealized Loss
Less Than 12 Months	(824,351)	(79,273)
12 Months or More	(1,296,119)	(601,185)
Total	(2,120,470)	(680,458)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	2,035,179	765,641
12 Months or More	358,521	408,460
Total	2,393,700	1,174,101
Unrealized Loss
Less Than 12 Months	(46,315)	(9,666)
12 Months or More	(19,548)	(13,064)
Total	(65,863)	(22,730)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	1,221,795	750,643
12 Months or More	732,139	779,086
Total	1,953,934	1,529,729
Unrealized Loss
Less Than 12 Months	(33,159)	(8,521)
12 Months or More	(38,401)	(21,031)
Total	(71,560)	(29,552)
Other asset-backed securities
Fair Value
Less Than 12 Months	363,599	86,506
12 Months or More	134,941	134,316
Total	498,540	220,822
Unrealized Loss
Less Than 12 Months	(2,966)	(322)
12 Months or More	(7,560)	(5,441)
Total	(10,526)	(5,763)
U.S. government-related securities
Fair Value
Less Than 12 Months	362,688	94,110
12 Months or More	1,046,716	1,072,232
Total	1,409,404	1,166,342
Unrealized Loss
Less Than 12 Months	(6,429)	(688)
12 Months or More	(50,863)	(31,489)
Total	(57,292)	(32,177)
Other government-related securities
Fair Value
Less Than 12 Months	176,212	24,830
12 Months or More	106,367	115,294
Total	282,579	140,124
Unrealized Loss
Less Than 12 Months	(7,250)	(169)
12 Months or More	(11,492)	(4,779)
Total	(18,742)	(4,948)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	985,821	170,268
12 Months or More	988,380	1,027,747
Total	1,974,201	1,198,015
Unrealized Loss
Less Than 12 Months	(20,690)	(1,738)
12 Months or More	(68,578)	(43,875)
Total	(89,268)	(45,613)
Corporate securities
Fair Value
Less Than 12 Months	20,298,992	5,026,417
12 Months or More	9,805,916	10,947,027
Total	30,104,908	15,973,444
Unrealized Loss
Less Than 12 Months	(705,423)	(55,649)
12 Months or More	(1,096,930)	(471,752)
Total	(1,802,353)	(527,401)
Redeemable preferred stock
Fair Value
Less Than 12 Months	56,339	22,048
12 Months or More	22,834	23,197
Total	79,173	45,245
Unrealized Loss
Less Than 12 Months	(2,119)	(1,120)
12 Months or More	(2,747)	(2,383)
Total	$ (4,866)	(3,503)
Equities
Fair Value
Less Than 12 Months		86,194
12 Months or More		91,195
Total		177,389
Unrealized Loss
Less Than 12 Months		(1,400)
12 Months or More		(7,371)
Total		$ (8,771)