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	$ 15,661,373	$ 7,062,734
12 Months or More	13,889,280	14,639,347
Total	29,550,653	21,702,081
Unrealized Loss
Less Than 12 Months	(412,923)	(79,496)
12 Months or More	(1,031,096)	(602,685)
Total	(1,444,019)	(682,181)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,426,703	766,599
12 Months or More	394,955	416,221
Total	1,821,658	1,182,820
Unrealized Loss
Less Than 12 Months	(29,466)	(9,671)
12 Months or More	(18,165)	(13,362)
Total	(47,631)	(23,033)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	984,307	757,471
12 Months or More	756,845	796,456
Total	1,741,152	1,553,927
Unrealized Loss
Less Than 12 Months	(24,232)	(8,592)
12 Months or More	(34,463)	(21,594)
Total	(58,695)	(30,186)
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	(1,784)	(322)
12 Months or More	(7,869)	(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)	(4,778)
Total	(11,626)	(4,947)
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)	(43,874)
Total	(73,487)	(45,612)
Corporate securities
Fair Value
Less Than 12 Months	12,307,646	5,054,316
12 Months or More	10,410,752	10,962,689
Total	22,718,398	16,017,005
Unrealized Loss
Less Than 12 Months	(337,936)	(55,795)
12 Months or More	(850,861)	(472,394)
Total	(1,188,797)	(528,189)
Redeemable 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		86,586
12 Months or More		91,195
Total		177,781
Unrealized Loss
Less Than 12 Months		(1,401)
12 Months or More		(7,370)
Total		$ (8,771)