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	Jun. 30, 2020	Dec. 31, 2019
Fair Value
Less Than 12 Months	$ 5,351,833	$ 5,345,197
12 Months or More	1,511,747	3,826,458
Total	6,863,580	9,171,655
Unrealized Loss
Less Than 12 Months	(281,471)	(85,800)
12 Months or More	(237,385)	(239,860)
Total	(518,856)	(325,660)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	113,660	851,333
12 Months or More	2,098	220,843
Total	115,758	1,072,176
Unrealized Loss
Less Than 12 Months	(498)	(4,231)
12 Months or More	(5)	(2,091)
Total	(503)	(6,322)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	412,551	371,945
12 Months or More	31,248	115,566
Total	443,799	487,511
Unrealized Loss
Less Than 12 Months	(27,826)	(1,721)
12 Months or More	(1,274)	(1,571)
Total	(29,100)	(3,292)
Other asset-backed securities
Fair Value
Less Than 12 Months	566,551	482,547
12 Months or More	282,432	214,058
Total	848,983	696,605
Unrealized Loss
Less Than 12 Months	(12,605)	(6,516)
12 Months or More	(15,499)	(8,410)
Total	(28,104)	(14,926)
U.S. government-related securities
Fair Value
Less Than 12 Months	134,265	383,451
12 Months or More	1,334	353,517
Total	135,599	736,968
Unrealized Loss
Less Than 12 Months	(815)	(3,373)
12 Months or More	(29)	(1,943)
Total	(844)	(5,316)
Other government-related securities
Fair Value
Less Than 12 Months	22,576	22,962
12 Months or More	6,358	6,230
Total	28,934	29,192
Unrealized Loss
Less Than 12 Months	(3,176)	(669)
12 Months or More	(1,185)	(1,322)
Total	(4,361)	(1,991)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	106,083	56,470
12 Months or More	5,965	12,907
Total	112,048	69,377
Unrealized Loss
Less Than 12 Months	(4,956)	(1,001)
12 Months or More	(77)	(229)
Total	(5,033)	(1,230)
Corporate securities
Fair Value
Less Than 12 Months	3,976,043	3,176,489
12 Months or More	1,182,312	2,886,648
Total	5,158,355	6,063,137
Unrealized Loss
Less Than 12 Months	(230,736)	(68,289)
12 Months or More	(219,316)	(220,045)
Total	(450,052)	(288,334)
Redeemable preferred stocks
Fair Value
Less Than 12 Months	20,104	0
12 Months or More	0	16,689
Total	20,104	16,689
Unrealized Loss
Less Than 12 Months	(859)	0
12 Months or More	0	(4,249)
Total	$ (859)	$ (4,249)