FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Available-for-sale securities	$ 37,785,148	$ 36,570,665
Total fixed maturity securities	39,374,923	38,183,337
Equity securities	787,317	754,489
Other long-term investments	429,369	417,769
Short-term investments	366,958	332,431
Assets related to separate accounts
Variable annuity	13,616,259	13,244,252
Variable universal life	959,638	895,925
Liabilities:
Other liabilities	203,002	176,979
Fixed maturities
Assets:
Available-for-sale securities	36,681,824	35,543,477
Trading securities	2,700,000	2,600,000
Residential mortgage-backed securities
Assets:
Available-for-sale securities	2,001,137	1,898,483
Commercial mortgage-backed securities
Assets:
Available-for-sale securities	1,832,129	1,811,470
Other asset-backed securities
Assets:
Available-for-sale securities	1,182,003	1,211,533
States, municipals, and political subdivisions
Assets:
Available-for-sale securities	1,689,321	1,656,503
Other government-related securities
Assets:
Available-for-sale securities	251,504	239,921
Corporate securities
Assets:
Available-for-sale securities	28,336,554	27,371,565
Level 3 | Other asset-backed securities
Assets:
Total investments	552,816	553,308
Level 3 | Corporate securities
Assets:
Total investments	634,340	638,279
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	39,374,923	38,183,337
Equity securities	787,317	754,489
Other long-term investments	549,541	542,243
Short-term investments	366,958	332,431
Total investments	41,078,739	39,812,500
Cash	404,871	348,182
Other assets	28,667	23,830
Assets related to separate accounts
Variable annuity	13,616,259	13,244,252
Variable universal life	959,638	895,925
Total assets measured at fair value on a recurring basis	56,088,174	54,324,689
Liabilities:
Annuity account balances	86,094	87,616
Other liabilities	905,220	748,821
Total liabilities measured at fair value on a recurring basis	991,314	836,437
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Available-for-sale securities	36,681,824	35,543,477
Trading securities	2,693,099	2,639,860
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Available-for-sale securities	2,001,137	1,898,483
Trading securities	264,109	255,027
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	1,832,129	1,811,470
Trading securities	152,522	149,683
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Available-for-sale securities	1,182,003	1,211,533
Trading securities	165,775	200,084
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Available-for-sale securities	1,296,108	1,268,159
Trading securities	26,101	26,961
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	1,689,321	1,656,503
Trading securities	323,720	316,519
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Available-for-sale securities	251,504	239,921
Trading securities	63,763	63,012
Measured at fair value on a recurring basis | Corporate securities
Assets:
Available-for-sale securities	28,336,554	27,371,565
Trading securities	1,693,996	1,624,589
Measured at fair value on a recurring basis | Preferred stock
Assets:
Available-for-sale securities	93,068	85,843
Trading securities	3,113	3,985
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,109,418	1,095,210
Equity securities	720,981	685,443
Other long-term investments	68,323	82,420
Short-term investments	336,860	328,829
Total investments	2,235,582	2,191,902
Cash	404,871	348,182
Other assets	28,667	23,830
Assets related to separate accounts
Variable annuity	13,616,259	13,244,252
Variable universal life	959,638	895,925
Total assets measured at fair value on a recurring basis	17,245,017	16,704,091
Liabilities:
Annuity account balances	0	0
Other liabilities	6,586	13,004
Total liabilities measured at fair value on a recurring basis	6,586	13,004
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Available-for-sale securities	1,084,696	1,068,801
Trading securities	24,722	26,409
Measured at fair value on a recurring basis | Level 1 | Residential mortgage-backed securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Other asset-backed securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | U.S. government-related securities
Assets:
Available-for-sale securities	1,010,562	1,002,020
Trading securities	21,609	22,424
Measured at fair value on a recurring basis | Level 1 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Other government-related securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Corporate securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Preferred stock
Assets:
Available-for-sale securities	74,134	66,781
Trading securities	3,113	3,985
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	36,977,583	35,771,419
Equity securities	36	36
Other long-term investments	361,195	335,498
Short-term investments	30,098	3,602
Total investments	37,368,912	36,110,555
Cash	0	0
Other assets	0	0
Assets related to separate accounts
Variable annuity	0	0
Variable universal life	0	0
Total assets measured at fair value on a recurring basis	37,368,912	36,110,555
Liabilities:
Annuity account balances	0	0
Other liabilities	196,416	163,974
Total liabilities measured at fair value on a recurring basis	196,416	163,974
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Available-for-sale securities	34,369,649	33,248,023
Trading securities	2,607,934	2,523,396
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Available-for-sale securities	1,989,275	1,898,480
Trading securities	264,109	255,027
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	1,832,129	1,811,470
Trading securities	152,522	149,683
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Available-for-sale securities	629,040	648,929
Trading securities	110,852	115,521
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Available-for-sale securities	285,546	266,139
Trading securities	4,492	4,537
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	1,689,321	1,656,503
Trading securities	323,720	316,519
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Available-for-sale securities	251,504	239,921
Trading securities	63,763	63,012
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Available-for-sale securities	27,673,900	26,707,519
Trading securities	1,688,476	1,619,097
Measured at fair value on a recurring basis | Level 2 | Preferred stock
Assets:
Available-for-sale securities	18,934	19,062
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,287,922	1,316,708
Equity securities	66,300	69,010
Other long-term investments	120,023	124,325
Short-term investments	0	0
Total investments	1,474,245	1,510,043
Cash	0	0
Other assets	0	0
Assets related to separate accounts
Variable annuity	0	0
Variable universal life	0	0
Total assets measured at fair value on a recurring basis	1,474,245	1,510,043
Liabilities:
Annuity account balances	86,094	87,616
Other liabilities	702,218	571,843
Total liabilities measured at fair value on a recurring basis	788,312	659,459
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Available-for-sale securities	1,227,479	1,226,653
Trading securities	60,443	90,055
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Available-for-sale securities	11,862	3
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Other asset-backed securities
Assets:
Available-for-sale securities	552,963	562,604
Trading securities	54,923	84,563
Measured at fair value on a recurring basis | Level 3 | U.S. government-related securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Other government-related securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Corporate securities
Assets:
Available-for-sale securities	662,654	664,046
Trading securities	5,520	5,492
Measured at fair value on a recurring basis | Level 3 | Preferred stock
Assets:
Available-for-sale securities	0	0
Trading securities	$ 0	$ 0