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, 2017	Dec. 31, 2016
Fair Value
Less Than 12 Months	$ 16,570,570	$ 18,169,517
12 Months or More	10,545,146	10,955,615
Total	27,115,716	29,125,132
Unrealized Loss
Less Than 12 Months	(562,670)	(663,606)
12 Months or More	(1,003,946)	(1,196,101)
Total	(1,566,616)	(1,859,707)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	1,128,994	1,051,694
12 Months or More	159,309	170,826
Total	1,288,303	1,222,520
Unrealized Loss
Less Than 12 Months	(24,690)	(21,178)
12 Months or More	(4,034)	(4,117)
Total	(28,724)	(25,295)
Commercial mortgage-backed securities
Fair Value
Less Than 12 Months	1,418,325	1,426,252
12 Months or More	97,152	100,475
Total	1,515,477	1,526,727
Unrealized Loss
Less Than 12 Months	(34,102)	(36,589)
12 Months or More	(4,324)	(4,013)
Total	(38,426)	(40,602)
Other asset-backed securities
Fair Value
Less Than 12 Months	260,723	323,706
12 Months or More	173,891	176,792
Total	434,614	500,498
Unrealized Loss
Less Than 12 Months	(5,836)	(9,291)
12 Months or More	(11,312)	(11,407)
Total	(17,148)	(20,698)
U.S. government-related securities
Fair Value
Less Than 12 Months	1,214,007	1,237,942
12 Months or More	2	3
Total	1,214,009	1,237,945
Unrealized Loss
Less Than 12 Months	(35,744)	(40,454)
12 Months or More	0	(1)
Total	(35,744)	(40,455)
Other government-related securities
Fair Value
Less Than 12 Months	71,542	98,412
12 Months or More	80,422	79,393
Total	151,964	177,805
Unrealized Loss
Less Than 12 Months	(1,267)	(2,907)
12 Months or More	(10,791)	(11,890)
Total	(12,058)	(14,797)
States, municipals, and political subdivisions
Fair Value
Less Than 12 Months	1,030,078	1,062,368
12 Months or More	546,377	548,254
Total	1,576,455	1,610,622
Unrealized Loss
Less Than 12 Months	(63,063)	(63,809)
12 Months or More	(42,740)	(41,749)
Total	(105,803)	(105,558)
Corporate securities
Fair Value
Less Than 12 Months	11,238,460	12,490,517
12 Months or More	9,398,629	9,791,313
Total	20,637,089	22,281,830
Unrealized Loss
Less Than 12 Months	(391,820)	(467,463)
12 Months or More	(923,263)	(1,114,635)
Total	(1,315,083)	(1,582,098)
Preferred Stock
Fair Value
Less Than 12 Months	59,654	66,781
12 Months or More	18,980	19,062
Total	78,634	85,843
Unrealized Loss
Less Than 12 Months	(3,446)	(6,642)
12 Months or More	(1,958)	(1,877)
Total	(5,404)	(8,519)
Equity securities
Fair Value
Less Than 12 Months	148,787	411,845
12 Months or More	70,384	69,497
Total	219,171	481,342
Unrealized Loss
Less Than 12 Months	(2,702)	(15,273)
12 Months or More	(5,524)	(6,412)
Total	$ (8,226)	$ (21,685)