INVESTMENT OPERATIONS - Schedule of Gross Unrealized Losses and Fair Value of Investments Not Deemed to be Other-Than-Temporary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value
Less Than 12 Months	$ 2,066,828	$ 5,345,197
12 Months or More	1,104,535	3,826,458
Total	3,171,363	9,171,655
Unrealized Loss
Less Than 12 Months	(54,577)	(85,800)
12 Months or More	(75,941)	(239,860)
Total	(130,518)	(325,660)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	386,013	851,333
12 Months or More	9,096	220,843
Total	395,109	1,072,176
Unrealized Loss
Less Than 12 Months	(935)	(4,231)
12 Months or More	(300)	(2,091)
Total	(1,235)	(6,322)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	262,752	371,945
12 Months or More	30,239	115,566
Total	292,991	487,511
Unrealized Loss
Less Than 12 Months	(15,623)	(1,721)
12 Months or More	(4,084)	(1,571)
Total	(19,707)	(3,292)
Other asset-backed securities
Fair Value
Less Than 12 Months	146,141	482,547
12 Months or More	326,132	214,058
Total	472,273	696,605
Unrealized Loss
Less Than 12 Months	(1,920)	(6,516)
12 Months or More	(4,794)	(8,410)
Total	(6,714)	(14,926)
U.S. government-related securities
Fair Value
Less Than 12 Months	162,293	383,451
12 Months or More	736	353,517
Total	163,029	736,968
Unrealized Loss
Less Than 12 Months	(1,678)	(3,373)
12 Months or More	(23)	(1,943)
Total	(1,701)	(5,316)
Other government-related securities
Fair Value
Less Than 12 Months	18,622	22,962
12 Months or More	6,975	6,230
Total	25,597	29,192
Unrealized Loss
Less Than 12 Months	(438)	(669)
12 Months or More	(559)	(1,322)
Total	(997)	(1,991)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	32,169	56,470
12 Months or More	4,876	12,907
Total	37,045	69,377
Unrealized Loss
Less Than 12 Months	(692)	(1,001)
12 Months or More	(57)	(229)
Total	(749)	(1,230)
Corporate securities
Fair Value
Less Than 12 Months	1,058,838	3,176,489
12 Months or More	726,481	2,886,648
Total	1,785,319	6,063,137
Unrealized Loss
Less Than 12 Months	(33,291)	(68,289)
12 Months or More	(66,124)	(220,045)
Total	(99,415)	(288,334)
Redeemable preferred stocks
Fair Value
Less Than 12 Months	0	0
12 Months or More	0	16,689
Total	0	16,689
Unrealized Loss
Less Than 12 Months	0	0
12 Months or More	0	(4,249)
Total	$ 0	$ (4,249)