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	$ 1,828,002	$ 22,322,949
12 Months or More	12,762,688	17,630,381
Total	14,590,690	39,953,330
Unrealized Loss
Less Than 12 Months	(59,528)	(1,091,842)
12 Months or More	(556,747)	(1,658,939)
Total	(616,275)	(2,750,781)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	118,932	1,485,009
12 Months or More	576,350	804,364
Total	695,282	2,289,373
Unrealized Loss
Less Than 12 Months	(427)	(31,302)
12 Months or More	(8,474)	(30,894)
Total	(8,901)	(62,196)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	3,609	422,438
12 Months or More	574,967	1,429,384
Total	578,576	1,851,822
Unrealized Loss
Less Than 12 Months	(6)	(7,442)
12 Months or More	(3,888)	(50,659)
Total	(3,894)	(58,101)
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	226,201
12 Months or More	93,790	131,569
Total	99,299	357,770
Unrealized Loss
Less Than 12 Months	(198)	(15,267)
12 Months or More	(3,970)	(18,583)
Total	(4,168)	(33,850)
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	1,244,536	18,326,331
12 Months or More	10,318,328	12,859,732
Total	11,562,864	31,186,063
Unrealized Loss
Less Than 12 Months	(52,639)	(970,553)
12 Months or More	(519,534)	(1,414,499)
Total	(572,173)	(2,385,052)
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)