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	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less Than 12 Months	$ 4,250,285	$ 22,322,949
12 Months or More	22,759,791	17,630,381
Total	27,010,076	39,953,330
Unrealized Loss
Less Than 12 Months	(153,656)	(1,091,842)
12 Months or More	(1,204,876)	(1,658,939)
Total	(1,358,532)	(2,750,781)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	158,901	1,485,009
12 Months or More	1,242,286	804,364
Total	1,401,187	2,289,373
Unrealized Loss
Less Than 12 Months	(1,772)	(31,302)
12 Months or More	(27,634)	(30,894)
Total	(29,406)	(62,196)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	52,747	422,438
12 Months or More	1,478,365	1,429,384
Total	1,531,112	1,851,822
Unrealized Loss
Less Than 12 Months	(1,235)	(7,442)
12 Months or More	(26,608)	(50,659)
Total	(27,843)	(58,101)
Other asset-backed securities
Fair Value
Less Than 12 Months	531,548	687,271
12 Months or More	192,201	148,871
Total	723,749	836,142
Unrealized Loss
Less Than 12 Months	(11,905)	(30,963)
12 Months or More	(3,718)	(4,435)
Total	(15,623)	(35,398)
U.S. government-related securities
Fair Value
Less Than 12 Months	43,977	130,290
12 Months or More	1,045,717	1,085,654
Total	1,089,694	1,215,944
Unrealized Loss
Less Than 12 Months	(640)	(4,668)
12 Months or More	(29,772)	(41,054)
Total	(30,412)	(45,722)
Other government-related securities
Fair Value
Less Than 12 Months	51,767	226,201
12 Months or More	198,112	131,569
Total	249,879	357,770
Unrealized Loss
Less Than 12 Months	(1,279)	(15,267)
12 Months or More	(10,604)	(18,583)
Total	(11,883)	(33,850)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	60,555	1,004,262
12 Months or More	911,555	1,129,152
Total	972,110	2,133,414
Unrealized Loss
Less Than 12 Months	(369)	(27,180)
12 Months or More	(24,887)	(91,722)
Total	(25,256)	(118,902)
Corporate securities
Fair Value
Less Than 12 Months	3,340,636	18,326,331
12 Months or More	17,623,264	12,859,732
Total	20,963,900	31,186,063
Unrealized Loss
Less Than 12 Months	(136,453)	(970,553)
12 Months or More	(1,077,532)	(1,414,499)
Total	(1,213,985)	(2,385,052)
Redeemable preferred stocks
Fair Value
Less Than 12 Months	10,154	41,147
12 Months or More	68,291	41,655
Total	78,445	82,802
Unrealized Loss
Less Than 12 Months	(3)	(4,467)
12 Months or More	(4,121)	(7,093)
Total	$ (4,124)	$ (11,560)