INVESTMENT OPERATIONS - Schedule of Gross Unrealized Losses and Fair Value of Investments Not Deemed to be Other-Than-Temporary (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 22,217,328	$ 7,026,657
12 Months or More	17,562,380	14,598,554
Total	39,779,708	21,625,211
Unrealized Loss
Less Than 12 Months	(1,088,010)	(79,273)
12 Months or More	(1,654,537)	(601,185)
Total	(2,742,547)	(680,458)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,485,009	765,641
12 Months or More	795,765	408,460
Total	2,280,774	1,174,101
Unrealized Loss
Less Than 12 Months	(31,302)	(9,666)
12 Months or More	(30,633)	(13,064)
Total	(61,935)	(22,730)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	419,420	750,643
12 Months or More	1,405,690	779,086
Total	1,825,110	1,529,729
Unrealized Loss
Less Than 12 Months	(7,398)	(8,521)
12 Months or More	(49,602)	(21,031)
Total	(57,000)	(29,552)
Other asset-backed securities
Fair Value
Less Than 12 Months	687,271	86,506
12 Months or More	148,871	134,316
Total	836,142	220,822
Unrealized Loss
Less Than 12 Months	(30,963)	(322)
12 Months or More	(4,435)	(5,441)
Total	(35,398)	(5,763)
U.S. government-related securities
Fair Value
Less Than 12 Months	130,290	94,110
12 Months or More	1,085,654	1,072,232
Total	1,215,944	1,166,342
Unrealized Loss
Less Than 12 Months	(4,668)	(688)
12 Months or More	(41,054)	(31,489)
Total	(45,722)	(32,177)
Other government-related securities
Fair Value
Less Than 12 Months	224,273	24,830
12 Months or More	131,569	115,294
Total	355,842	140,124
Unrealized Loss
Less Than 12 Months	(15,207)	(169)
12 Months or More	(18,583)	(4,779)
Total	(33,790)	(4,948)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	1,004,262	170,268
12 Months or More	1,129,152	1,027,747
Total	2,133,414	1,198,015
Unrealized Loss
Less Than 12 Months	(27,180)	(1,738)
12 Months or More	(91,722)	(43,875)
Total	(118,902)	(45,613)
Corporate securities
Fair Value
Less Than 12 Months	18,225,656	5,026,417
12 Months or More	12,824,024	10,947,027
Total	31,049,680	15,973,444
Unrealized Loss
Less Than 12 Months	(966,825)	(55,649)
12 Months or More	(1,411,415)	(471,752)
Total	(2,378,240)	(527,401)
Redeemable preferred stock
Fair Value
Less Than 12 Months	41,147	22,048
12 Months or More	41,655	23,197
Total	82,802	45,245
Unrealized Loss
Less Than 12 Months	(4,467)	(1,120)
12 Months or More	(7,093)	(2,383)
Total	$ (11,560)	(3,503)
Equities
Fair Value
Less Than 12 Months		86,194
12 Months or More		91,195
Total		177,389
Unrealized Loss
Less Than 12 Months		(1,400)
12 Months or More		(7,371)
Total		$ (8,771)