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	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 22,322,949	$ 7,062,734
12 Months or More	17,630,381	14,639,347
Total	39,953,330	21,702,081
Unrealized Loss
Less Than 12 Months	(1,091,842)	(79,496)
12 Months or More	(1,658,939)	(602,685)
Total	(2,750,781)	(682,181)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,485,009	766,599
12 Months or More	804,364	416,221
Total	2,289,373	1,182,820
Unrealized Loss
Less Than 12 Months	(31,302)	(9,671)
12 Months or More	(30,894)	(13,362)
Total	(62,196)	(23,033)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	422,438	757,471
12 Months or More	1,429,384	796,456
Total	1,851,822	1,553,927
Unrealized Loss
Less Than 12 Months	(7,442)	(8,592)
12 Months or More	(50,659)	(21,594)
Total	(58,101)	(30,186)
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	226,201	24,830
12 Months or More	131,569	115,294
Total	357,770	140,124
Unrealized Loss
Less Than 12 Months	(15,267)	(169)
12 Months or More	(18,583)	(4,778)
Total	(33,850)	(4,947)
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)	(43,874)
Total	(118,902)	(45,612)
Corporate securities
Fair Value
Less Than 12 Months	18,326,331	5,054,316
12 Months or More	12,859,732	10,962,689
Total	31,186,063	16,017,005
Unrealized Loss
Less Than 12 Months	(970,553)	(55,795)
12 Months or More	(1,414,499)	(472,394)
Total	(2,385,052)	(528,189)
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)
Equity securities
Fair Value
Less Than 12 Months		86,586
12 Months or More		91,195
Total		177,781
Unrealized Loss
Less Than 12 Months		(1,401)
12 Months or More		(7,370)
Total		$ (8,771)