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, 2019	Dec. 31, 2018
Fair Value
Less Than 12 Months	$ 1,819,092	$ 22,217,328
12 Months or More	12,730,048	17,562,380
Total	14,549,140	39,779,708
Unrealized Loss
Less Than 12 Months	(59,465)	(1,088,010)
12 Months or More	(555,975)	(1,654,537)
Total	(615,440)	(2,742,547)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	113,964	1,485,009
12 Months or More	572,514	795,765
Total	686,478	2,280,774
Unrealized Loss
Less Than 12 Months	(413)	(31,302)
12 Months or More	(8,469)	(30,633)
Total	(8,882)	(61,935)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	3,609	419,420
12 Months or More	563,319	1,405,690
Total	566,928	1,825,110
Unrealized Loss
Less Than 12 Months	(6)	(7,398)
12 Months or More	(3,811)	(49,602)
Total	(3,817)	(57,000)
Other asset-backed securities
Fair Value
Less Than 12 Months	409,753	687,271
12 Months or More	134,156	148,871
Total	543,909	836,142
Unrealized Loss
Less Than 12 Months	(5,931)	(30,963)
12 Months or More	(2,595)	(4,435)
Total	(8,526)	(35,398)
U.S. government-related securities
Fair Value
Less Than 12 Months	32,421	130,290
12 Months or More	902,249	1,085,654
Total	934,670	1,215,944
Unrealized Loss
Less Than 12 Months	(314)	(4,668)
12 Months or More	(8,690)	(41,054)
Total	(9,004)	(45,722)
Other government-related securities
Fair Value
Less Than 12 Months	5,509	224,273
12 Months or More	93,790	131,569
Total	99,299	355,842
Unrealized Loss
Less Than 12 Months	(198)	(15,207)
12 Months or More	(3,970)	(18,583)
Total	(4,168)	(33,790)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	13,242	1,004,262
12 Months or More	145,668	1,129,152
Total	158,910	2,133,414
Unrealized Loss
Less Than 12 Months	(13)	(27,180)
12 Months or More	(5,838)	(91,722)
Total	(5,851)	(118,902)
Corporate securities
Fair Value
Less Than 12 Months	1,240,594	18,225,656
12 Months or More	10,301,172	12,824,024
Total	11,541,766	31,049,680
Unrealized Loss
Less Than 12 Months	(52,590)	(966,825)
12 Months or More	(518,844)	(1,411,415)
Total	(571,434)	(2,378,240)
Redeemable preferred stocks
Fair Value
Less Than 12 Months	0	41,147
12 Months or More	17,180	41,655
Total	17,180	82,802
Unrealized Loss
Less Than 12 Months	0	(4,467)
12 Months or More	(3,758)	(7,093)
Total	$ (3,758)	$ (11,560)