FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Fixed maturity securities - available-for-sale	$ 39,356,087	$ 36,570,665
Total fixed maturity securities	40,777,816	38,183,337
Equity securities	781,537	754,489
Other long-term investments	441,393	417,769
Short-term investments	527,179	332,431
Assets related to separate accounts
Variable annuity	13,763,971	13,244,252
Variable universal life	996,375	895,925
Liabilities:
Other liabilities	230,082	176,979
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	38,096,737	35,543,477
Fixed maturity securities - trading	2,700,000	2,600,000
Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,192,254	1,898,483
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,921,429	1,811,470
Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,190,667	1,211,533
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	1,704,009	1,656,503
Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	254,336	239,921
Corporate securities
Assets:
Fixed maturity securities - available-for-sale	29,449,369	27,371,565
Level 3 | Other asset-backed securities
Assets:
Total investments	553,005	553,308
Level 3 | Corporate securities
Assets:
Total investments	615,576	638,279
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	40,777,816	38,183,337
Equity securities	781,537	754,489
Other long-term investments	571,936	542,243
Short-term investments	527,179	332,431
Total investments	42,658,468	39,812,500
Cash	257,562	348,182
Other assets	29,927	23,830
Assets related to separate accounts
Variable annuity	13,763,971	13,244,252
Variable universal life	996,375	895,925
Total assets measured at fair value on a recurring basis	57,706,303	54,324,689
Liabilities:
Annuity account balances	85,028	87,616
Other liabilities	966,495	748,821
Total liabilities measured at fair value on a recurring basis	1,051,523	836,437
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	38,096,737	35,543,477
Fixed maturity securities - trading	2,681,079	2,639,860
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,192,254	1,898,483
Fixed maturity securities - trading	262,062	255,027
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,921,429	1,811,470
Fixed maturity securities - trading	145,653	149,683
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,190,667	1,211,533
Fixed maturity securities - trading	142,613	200,084
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,292,833	1,268,159
Fixed maturity securities - trading	27,714	26,961
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	1,704,009	1,656,503
Fixed maturity securities - trading	321,001	316,519
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	254,336	239,921
Fixed maturity securities - trading	64,067	63,012
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	29,449,369	27,371,565
Fixed maturity securities - trading	1,714,740	1,624,589
Measured at fair value on a recurring basis | Preferred stock
Assets:
Fixed maturity securities - available-for-sale	91,840	85,843
Fixed maturity securities - trading	3,229	3,985
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,106,985	1,095,210
Equity securities	715,339	685,443
Other long-term investments	61,072	82,420
Short-term investments	421,408	328,829
Total investments	2,304,804	2,191,902
Cash	257,562	348,182
Other assets	29,927	23,830
Assets related to separate accounts
Variable annuity	13,763,971	13,244,252
Variable universal life	996,375	895,925
Total assets measured at fair value on a recurring basis	17,352,639	16,704,091
Liabilities:
Annuity account balances	0	0
Other liabilities	23,775	13,004
Total liabilities measured at fair value on a recurring basis	23,775	13,004
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,082,167	1,068,801
Fixed maturity securities - trading	24,818	26,409
Measured at fair value on a recurring basis | Level 1 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,009,225	1,002,020
Fixed maturity securities - trading	21,589	22,424
Measured at fair value on a recurring basis | Level 1 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | Preferred stock
Assets:
Fixed maturity securities - available-for-sale	72,942	66,781
Fixed maturity securities - trading	3,229	3,985
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	38,452,115	35,771,419
Equity securities	36	36
Other long-term investments	380,512	335,498
Short-term investments	105,771	3,602
Total investments	38,938,434	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	38,938,434	36,110,555
Liabilities:
Annuity account balances	0	0
Other liabilities	206,307	163,974
Total liabilities measured at fair value on a recurring basis	206,307	163,974
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	35,836,868	33,248,023
Fixed maturity securities - trading	2,615,247	2,523,396
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,192,254	1,898,480
Fixed maturity securities - trading	262,062	255,027
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,921,429	1,811,470
Fixed maturity securities - trading	145,653	149,683
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	637,518	648,929
Fixed maturity securities - trading	107,123	115,521
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	283,608	266,139
Fixed maturity securities - trading	6,125	4,537
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	1,704,009	1,656,503
Fixed maturity securities - trading	321,001	316,519
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	254,336	239,921
Fixed maturity securities - trading	64,067	63,012
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	28,824,816	26,707,519
Fixed maturity securities - trading	1,709,216	1,619,097
Measured at fair value on a recurring basis | Level 2 | Preferred stock
Assets:
Fixed maturity securities - available-for-sale	18,898	19,062
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,218,716	1,316,708
Equity securities	66,162	69,010
Other long-term investments	130,352	124,325
Short-term investments	0	0
Total investments	1,415,230	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	1,415,230	1,510,043
Liabilities:
Annuity account balances	85,028	87,616
Other liabilities	736,413	571,843
Total liabilities measured at fair value on a recurring basis	821,441	659,459
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,177,702	1,226,653
Fixed maturity securities - trading	41,014	90,055
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	0	3
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	553,149	562,604
Fixed maturity securities - trading	35,490	84,563
Measured at fair value on a recurring basis | Level 3 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	624,553	664,046
Fixed maturity securities - trading	5,524	5,492
Measured at fair value on a recurring basis | Level 3 | Preferred stock
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	$ 0	$ 0