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, 2019	Dec. 31, 2018
Fair Value
Less Than 12 Months	$ 5,345,197	$ 22,217,328
12 Months or More	3,826,458	17,562,380
Total	9,171,655	39,779,708
Unrealized Loss
Less Than 12 Months	(85,800)	(1,088,010)
12 Months or More	(239,860)	(1,654,537)
Total	(325,660)	(2,742,547)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	851,333	1,485,009
12 Months or More	220,843	795,765
Total	1,072,176	2,280,774
Unrealized Loss
Less Than 12 Months	(4,231)	(31,302)
12 Months or More	(2,091)	(30,633)
Total	(6,322)	(61,935)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	371,945	419,420
12 Months or More	115,566	1,405,690
Total	487,511	1,825,110
Unrealized Loss
Less Than 12 Months	(1,721)	(7,398)
12 Months or More	(1,571)	(49,602)
Total	(3,292)	(57,000)
Other asset-backed securities
Fair Value
Less Than 12 Months	482,547	687,271
12 Months or More	214,058	148,871
Total	696,605	836,142
Unrealized Loss
Less Than 12 Months	(6,516)	(30,963)
12 Months or More	(8,410)	(4,435)
Total	(14,926)	(35,398)
U.S. government-related securities
Fair Value
Less Than 12 Months	383,451	130,290
12 Months or More	353,517	1,085,654
Total	736,968	1,215,944
Unrealized Loss
Less Than 12 Months	(3,373)	(4,668)
12 Months or More	(1,943)	(41,054)
Total	(5,316)	(45,722)
Other government-related securities
Fair Value
Less Than 12 Months	22,962	224,273
12 Months or More	6,230	131,569
Total	29,192	355,842
Unrealized Loss
Less Than 12 Months	(669)	(15,207)
12 Months or More	(1,322)	(18,583)
Total	(1,991)	(33,790)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	56,470	1,004,262
12 Months or More	12,907	1,129,152
Total	69,377	2,133,414
Unrealized Loss
Less Than 12 Months	(1,001)	(27,180)
12 Months or More	(229)	(91,722)
Total	(1,230)	(118,902)
Corporate securities
Fair Value
Less Than 12 Months	3,176,489	18,225,656
12 Months or More	2,886,648	12,824,024
Total	6,063,137	31,049,680
Unrealized Loss
Less Than 12 Months	(68,289)	(966,825)
12 Months or More	(220,045)	(1,411,415)
Total	(288,334)	(2,378,240)
Redeemable preferred stocks
Fair Value
Less Than 12 Months	0	41,147
12 Months or More	16,689	41,655
Total	16,689	82,802
Unrealized Loss
Less Than 12 Months	0	(4,467)
12 Months or More	(4,249)	(7,093)
Total	$ (4,249)	$ (11,560)