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	Dec. 31, 2017	Dec. 31, 2016
Fair Value
Less Than 12 Months	$ 7,062,734	$ 18,267,283
12 Months or More	14,639,347	10,957,881
Total	21,702,081	29,225,164
Unrealized Loss
Less Than 12 Months	(79,496)	(666,780)
12 Months or More	(602,685)	(1,196,195)
Total	(682,181)	(1,862,975)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	766,599	1,060,569
12 Months or More	416,221	170,826
Total	1,182,820	1,231,395
Unrealized Loss
Less Than 12 Months	(9,671)	(21,550)
12 Months or More	(13,362)	(4,117)
Total	(23,033)	(25,667)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	757,471	1,452,146
12 Months or More	796,456	100,475
Total	1,553,927	1,552,621
Unrealized Loss
Less Than 12 Months	(8,592)	(37,665)
12 Months or More	(21,594)	(4,013)
Total	(30,186)	(41,678)
Other asset-backed securities
Fair Value
Less Than 12 Months	86,506	323,706
12 Months or More	134,316	176,792
Total	220,822	500,498
Unrealized Loss
Less Than 12 Months	(322)	(9,291)
12 Months or More	(5,441)	(11,407)
Total	(5,763)	(20,698)
U.S. government-related securities
Fair Value
Less Than 12 Months	94,110	1,237,942
12 Months or More	1,072,232	3
Total	1,166,342	1,237,945
Unrealized Loss
Less Than 12 Months	(688)	(40,454)
12 Months or More	(31,489)	(1)
Total	(32,177)	(40,455)
Other government-related securities
Fair Value
Less Than 12 Months	24,830	98,412
12 Months or More	115,294	79,393
Total	140,124	177,805
Unrealized Loss
Less Than 12 Months	(169)	(2,907)
12 Months or More	(4,778)	(11,890)
Total	(4,947)	(14,797)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	170,268	1,062,368
12 Months or More	1,027,747	548,254
Total	1,198,015	1,610,622
Unrealized Loss
Less Than 12 Months	(1,738)	(63,809)
12 Months or More	(43,874)	(41,749)
Total	(45,612)	(105,558)
Corporate securities
Fair Value
Less Than 12 Months	5,054,316	12,553,514
12 Months or More	10,962,689	9,793,579
Total	16,017,005	22,347,093
Unrealized Loss
Less Than 12 Months	(55,795)	(469,189)
12 Months or More	(472,394)	(1,114,729)
Total	(528,189)	(1,583,918)
Redeemable preferred stock
Fair Value
Less Than 12 Months	22,048	66,781
12 Months or More	23,197	19,062
Total	45,245	85,843
Unrealized Loss
Less Than 12 Months	(1,120)	(6,642)
12 Months or More	(2,383)	(1,877)
Total	(3,503)	(8,519)
Equity securities
Fair Value
Less Than 12 Months	86,586	411,845
12 Months or More	91,195	69,497
Total	177,781	481,342
Unrealized Loss
Less Than 12 Months	(1,401)	(15,273)
12 Months or More	(7,370)	(6,412)
Total	$ (8,771)	$ (21,685)