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	Mar. 31, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 15,571,799	$ 7,026,657
12 Months or More	13,849,110	14,598,554
Total	29,420,909	21,625,211
Unrealized Loss
Less Than 12 Months	(410,760)	(79,273)
12 Months or More	(1,028,991)	(601,185)
Total	(1,439,751)	(680,458)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,425,844	765,641
12 Months or More	387,102	408,460
Total	1,812,946	1,174,101
Unrealized Loss
Less Than 12 Months	(29,450)	(9,666)
12 Months or More	(17,961)	(13,064)
Total	(47,411)	(22,730)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	975,588	750,643
12 Months or More	739,811	779,086
Total	1,715,399	1,529,729
Unrealized Loss
Less Than 12 Months	(24,006)	(8,521)
12 Months or More	(33,580)	(21,031)
Total	(57,586)	(29,552)
Other asset-backed securities
Fair Value
Less Than 12 Months	143,917	86,506
12 Months or More	129,840	134,316
Total	273,757	220,822
Unrealized Loss
Less Than 12 Months	(1,783)	(322)
12 Months or More	(7,870)	(5,441)
Total	(9,653)	(5,763)
U.S. government-related securities
Fair Value
Less Than 12 Months	145,876	94,110
12 Months or More	1,054,322	1,072,232
Total	1,200,198	1,166,342
Unrealized Loss
Less Than 12 Months	(3,108)	(688)
12 Months or More	(46,893)	(31,489)
Total	(50,001)	(32,177)
Other government-related securities
Fair Value
Less Than 12 Months	90,010	24,830
12 Months or More	112,461	115,294
Total	202,471	140,124
Unrealized Loss
Less Than 12 Months	(4,221)	(169)
12 Months or More	(7,405)	(4,779)
Total	(11,626)	(4,948)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	505,864	170,268
12 Months or More	1,007,246	1,027,747
Total	1,513,110	1,198,015
Unrealized Loss
Less Than 12 Months	(10,768)	(1,738)
12 Months or More	(62,719)	(43,875)
Total	(73,487)	(45,613)
Corporate securities
Fair Value
Less Than 12 Months	12,227,650	5,026,417
12 Months or More	10,395,469	10,947,027
Total	22,623,119	15,973,444
Unrealized Loss
Less Than 12 Months	(336,016)	(55,649)
12 Months or More	(849,842)	(471,752)
Total	(1,185,858)	(527,401)
Preferred Stock
Fair Value
Less Than 12 Months	57,050	22,048
12 Months or More	22,859	23,197
Total	79,909	45,245
Unrealized Loss
Less Than 12 Months	(1,408)	(1,120)
12 Months or More	(2,721)	(2,383)
Total	$ (4,129)	(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)