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, 2020	Dec. 31, 2019
Fair Value
Less Than 12 Months	$ 4,138,572	$ 5,345,197
12 Months or More	1,436,399	3,826,458
Total	5,574,971	9,171,655
Unrealized Loss
Less Than 12 Months	(188,570)	(85,800)
12 Months or More	(156,207)	(239,860)
Total	(344,777)	(325,660)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	176,534	851,333
12 Months or More	6,895	220,843
Total	183,429	1,072,176
Unrealized Loss
Less Than 12 Months	(653)	(4,231)
12 Months or More	(236)	(2,091)
Total	(889)	(6,322)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	354,926	371,945
12 Months or More	28,103	115,566
Total	383,029	487,511
Unrealized Loss
Less Than 12 Months	(21,686)	(1,721)
12 Months or More	(5,751)	(1,571)
Total	(27,437)	(3,292)
Other asset-backed securities
Fair Value
Less Than 12 Months	379,311	482,547
12 Months or More	312,368	214,058
Total	691,679	696,605
Unrealized Loss
Less Than 12 Months	(6,209)	(6,516)
12 Months or More	(11,547)	(8,410)
Total	(17,756)	(14,926)
U.S. government-related securities
Fair Value
Less Than 12 Months	251,534	383,451
12 Months or More	787	353,517
Total	252,321	736,968
Unrealized Loss
Less Than 12 Months	(3,193)	(3,373)
12 Months or More	(26)	(1,943)
Total	(3,219)	(5,316)
Other government-related securities
Fair Value
Less Than 12 Months	23,328	22,962
12 Months or More	6,776	6,230
Total	30,104	29,192
Unrealized Loss
Less Than 12 Months	(1,900)	(669)
12 Months or More	(762)	(1,322)
Total	(2,662)	(1,991)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	105,894	56,470
12 Months or More	4,895	12,907
Total	110,789	69,377
Unrealized Loss
Less Than 12 Months	(2,829)	(1,001)
12 Months or More	(52)	(229)
Total	(2,881)	(1,230)
Corporate securities
Fair Value
Less Than 12 Months	2,847,045	3,176,489
12 Months or More	1,076,575	2,886,648
Total	3,923,620	6,063,137
Unrealized Loss
Less Than 12 Months	(152,100)	(68,289)
12 Months or More	(137,833)	(220,045)
Total	(289,933)	(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)