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	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 29,871,063	$ 7,062,734
12 Months or More	13,226,886	14,639,347
Total	43,097,949	21,702,081
Unrealized Loss
Less Than 12 Months	(991,547)	(79,496)
12 Months or More	(1,379,164)	(602,685)
Total	(2,370,711)	(682,181)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	2,831,969	766,599
12 Months or More	407,345	416,221
Total	3,239,314	1,182,820
Unrealized Loss
Less Than 12 Months	(80,873)	(9,671)
12 Months or More	(25,077)	(13,362)
Total	(105,950)	(23,033)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	1,442,862	757,471
12 Months or More	757,095	796,456
Total	2,199,957	1,553,927
Unrealized Loss
Less Than 12 Months	(40,080)	(8,592)
12 Months or More	(41,983)	(21,594)
Total	(82,063)	(30,186)
Other asset-backed securities
Fair Value
Less Than 12 Months	645,956	86,506
12 Months or More	125,847	134,316
Total	771,803	220,822
Unrealized Loss
Less Than 12 Months	(9,131)	(322)
12 Months or More	(7,251)	(5,441)
Total	(16,382)	(5,763)
U.S. government-related securities
Fair Value
Less Than 12 Months	551,939	94,110
12 Months or More	1,012,450	1,072,232
Total	1,564,389	1,166,342
Unrealized Loss
Less Than 12 Months	(14,066)	(688)
12 Months or More	(58,741)	(31,489)
Total	(72,807)	(32,177)
Other government-related securities
Fair Value
Less Than 12 Months	256,902	24,830
12 Months or More	111,089	115,294
Total	367,991	140,124
Unrealized Loss
Less Than 12 Months	(7,459)	(169)
12 Months or More	(14,229)	(4,778)
Total	(21,688)	(4,947)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	2,268,935	170,268
12 Months or More	998,399	1,027,747
Total	3,267,334	1,198,015
Unrealized Loss
Less Than 12 Months	(51,019)	(1,738)
12 Months or More	(88,908)	(43,874)
Total	(139,927)	(45,612)
Corporate securities
Fair Value
Less Than 12 Months	21,828,788	5,054,316
12 Months or More	9,771,234	10,962,689
Total	31,600,022	16,017,005
Unrealized Loss
Less Than 12 Months	(787,017)	(55,795)
12 Months or More	(1,137,654)	(472,394)
Total	(1,924,671)	(528,189)
Redeemable preferred stock
Fair Value
Less Than 12 Months	43,712	22,048
12 Months or More	43,427	23,197
Total	87,139	45,245
Unrealized Loss
Less Than 12 Months	(1,902)	(1,120)
12 Months or More	(5,321)	(2,383)
Total	$ (7,223)	(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)