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, 2019	Dec. 31, 2018
Fair Value
Less Than 12 Months	$ 3,497,402	$ 22,217,328
12 Months or More	4,570,116	17,562,380
Total	8,067,518	39,779,708
Unrealized Loss
Less Than 12 Months	(55,170)	(1,088,010)
12 Months or More	(296,147)	(1,654,537)
Total	(351,317)	(2,742,547)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	484,979	1,485,009
12 Months or More	287,356	795,765
Total	772,335	2,280,774
Unrealized Loss
Less Than 12 Months	(2,069)	(31,302)
12 Months or More	(2,951)	(30,633)
Total	(5,020)	(61,935)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	67,255	419,420
12 Months or More	117,799	1,405,690
Total	185,054	1,825,110
Unrealized Loss
Less Than 12 Months	(78)	(7,398)
12 Months or More	(1,041)	(49,602)
Total	(1,119)	(57,000)
Other asset-backed securities
Fair Value
Less Than 12 Months	453,923	687,271
12 Months or More	162,369	148,871
Total	616,292	836,142
Unrealized Loss
Less Than 12 Months	(7,510)	(30,963)
12 Months or More	(5,507)	(4,435)
Total	(13,017)	(35,398)
U.S. government-related securities
Fair Value
Less Than 12 Months	265,257	130,290
12 Months or More	335,336	1,085,654
Total	600,593	1,215,944
Unrealized Loss
Less Than 12 Months	(844)	(4,668)
12 Months or More	(1,725)	(41,054)
Total	(2,569)	(45,722)
Other government-related securities
Fair Value
Less Than 12 Months	57,008	224,273
12 Months or More	10,427	131,569
Total	67,435	355,842
Unrealized Loss
Less Than 12 Months	(488)	(15,207)
12 Months or More	(1,142)	(18,583)
Total	(1,630)	(33,790)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	35,092	1,004,262
12 Months or More	15,215	1,129,152
Total	50,307	2,133,414
Unrealized Loss
Less Than 12 Months	(297)	(27,180)
12 Months or More	(300)	(91,722)
Total	(597)	(118,902)
Corporate securities
Fair Value
Less Than 12 Months	2,133,888	18,225,656
12 Months or More	3,624,679	12,824,024
Total	5,758,567	31,049,680
Unrealized Loss
Less Than 12 Months	(43,884)	(966,825)
12 Months or More	(279,478)	(1,411,415)
Total	(323,362)	(2,378,240)
Redeemable preferred stocks
Fair Value
Less Than 12 Months	0	41,147
12 Months or More	16,935	41,655
Total	16,935	82,802
Unrealized Loss
Less Than 12 Months	0	(4,467)
12 Months or More	(4,003)	(7,093)
Total	$ (4,003)	$ (11,560)