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	Sep. 30, 2017	Dec. 31, 2016
Fair Value
Less Than 12 Months	$ 12,469,365	$ 18,267,283
12 Months or More	9,776,361	10,957,881
Total	22,245,726	29,225,164
Unrealized Loss
Less Than 12 Months	(238,548)	(666,780)
12 Months or More	(624,633)	(1,196,195)
Total	(863,181)	(1,862,975)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,044,494	1,060,569
12 Months or More	88,573	170,826
Total	1,133,067	1,231,395
Unrealized Loss
Less Than 12 Months	(15,210)	(21,550)
12 Months or More	(2,137)	(4,117)
Total	(17,347)	(25,667)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	1,282,643	1,452,146
12 Months or More	159,684	100,475
Total	1,442,327	1,552,621
Unrealized Loss
Less Than 12 Months	(19,936)	(37,665)
12 Months or More	(4,634)	(4,013)
Total	(24,570)	(41,678)
Other asset-backed securities
Fair Value
Less Than 12 Months	202,097	323,706
12 Months or More	170,747	176,792
Total	372,844	500,498
Unrealized Loss
Less Than 12 Months	(5,064)	(9,291)
12 Months or More	(8,424)	(11,407)
Total	(13,488)	(20,698)
U.S. government-related securities
Fair Value
Less Than 12 Months	1,090,531	1,237,942
12 Months or More	85,948	3
Total	1,176,479	1,237,945
Unrealized Loss
Less Than 12 Months	(21,099)	(40,454)
12 Months or More	(5,234)	(1)
Total	(26,333)	(40,455)
Other government-related securities
Fair Value
Less Than 12 Months	44,015	98,412
12 Months or More	84,524	79,393
Total	128,539	177,805
Unrealized Loss
Less Than 12 Months	(287)	(2,907)
12 Months or More	(6,548)	(11,890)
Total	(6,835)	(14,797)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	819,311	1,062,368
12 Months or More	615,181	548,254
Total	1,434,492	1,610,622
Unrealized Loss
Less Than 12 Months	(22,392)	(63,809)
12 Months or More	(36,880)	(41,749)
Total	(59,272)	(105,558)
Corporate securities
Fair Value
Less Than 12 Months	7,825,478	12,553,514
12 Months or More	8,487,852	9,793,579
Total	16,313,330	22,347,093
Unrealized Loss
Less Than 12 Months	(151,509)	(469,189)
12 Months or More	(553,638)	(1,114,729)
Total	(705,147)	(1,583,918)
Preferred stock
Fair Value
Less Than 12 Months	52,294	66,781
12 Months or More	28,846	19,062
Total	81,140	85,843
Unrealized Loss
Less Than 12 Months	(806)	(6,642)
12 Months or More	(2,092)	(1,877)
Total	(2,898)	(8,519)
Equities
Fair Value
Less Than 12 Months	108,502	411,845
12 Months or More	55,006	69,497
Total	163,508	481,342
Unrealized Loss
Less Than 12 Months	(2,245)	(15,273)
12 Months or More	(5,046)	(6,412)
Total	$ (7,291)	$ (21,685)