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:
Available-for-sale securities, fair value	$ 39,007,653	$ 36,328,836
Total fixed maturity securities	40,571,237	37,996,577
Equity securities	744,176	716,017
Other long-term investments	441,393	417,769
Short-term investments	422,685	315,834
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:
Available-for-sale securities, fair value	37,890,158	35,356,717
Trading securities	2,700,000	2,600,000
Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	2,183,369	1,889,607
Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,892,053	1,782,497
Other asset-backed securities
Assets:
Available-for-sale securities, fair value	1,190,667	1,211,533
States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,704,009	1,656,503
Other government-related securities
Assets:
Available-for-sale securities, fair value	252,280	237,926
Corporate securities
Assets:
Available-for-sale securities, fair value	29,283,107	27,224,649
Level 3 | Other asset-backed securities
Assets related to separate accounts
Total assets measured at fair value on a recurring basis	553,005	553,308
Level 3 | Corporate securities
Assets related to separate accounts
Total assets measured at fair value on a recurring basis	615,576	638,279
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	40,571,237	37,996,577
Equity securities	744,176	716,017
Other long-term investments	591,317	533,433
Short-term investments	422,685	315,834
Total investments	42,329,415	39,561,861
Cash	199,633	214,439
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	57,289,394	53,916,477
Liabilities:
Annuity account balances	85,028	87,616
Other liabilities	841,900	674,048
Total liabilities measured at fair value on a recurring basis	926,928	761,664
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Available-for-sale securities, fair value	37,890,158	35,356,717
Trading securities	2,681,079	2,639,860
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	2,183,369	1,889,607
Trading securities	262,062	255,027
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,892,053	1,782,497
Trading securities	145,653	149,683
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	1,190,667	1,211,533
Trading securities	142,613	200,084
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	1,292,833	1,268,159
Trading securities	27,714	26,961
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,704,009	1,656,503
Trading securities	321,001	316,519
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Available-for-sale securities, fair value	252,280	237,926
Trading securities	64,067	63,012
Measured at fair value on a recurring basis | Corporate securities
Assets:
Available-for-sale securities, fair value	29,283,107	27,224,649
Trading securities	1,714,740	1,624,589
Measured at fair value on a recurring basis | Preferred stock
Assets:
Available-for-sale securities, fair value	91,840	85,843
Trading securities	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	678,660	650,231
Other long-term investments	61,072	82,420
Short-term investments	316,914	313,835
Total investments	2,163,631	2,141,696
Cash	199,633	214,439
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	17,123,610	16,496,312
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:
Available-for-sale securities, fair value	1,082,167	1,068,801
Trading securities	24,818	26,409
Measured at fair value on a recurring basis | Level 1 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	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, fair value	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, fair value	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, fair value	1,009,225	1,002,020
Trading securities	21,589	22,424
Measured at fair value on a recurring basis | Level 1 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	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, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Corporate securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Preferred stock
Assets:
Available-for-sale securities, fair value	72,942	66,781
Trading securities	3,229	3,985
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	38,245,536	35,584,659
Equity securities	0	0
Other long-term investments	380,511	335,497
Short-term investments	105,771	1,999
Total investments	38,731,818	35,922,155
Cash	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	38,731,818	35,922,155
Liabilities:
Annuity account balances	0	0
Other liabilities	256,829	255,241
Total liabilities measured at fair value on a recurring basis	256,829	255,241
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Available-for-sale securities, fair value	35,630,289	33,061,263
Trading securities	2,615,247	2,523,396
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	2,183,369	1,889,604
Trading securities	262,062	255,027
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,892,053	1,782,497
Trading securities	145,653	149,683
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	637,518	648,929
Trading securities	107,123	115,521
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	283,608	266,139
Trading securities	6,125	4,537
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,704,009	1,656,503
Trading securities	321,001	316,519
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Available-for-sale securities, fair value	252,280	237,926
Trading securities	64,067	63,012
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Available-for-sale securities, fair value	28,658,554	26,560,603
Trading securities	1,709,216	1,619,097
Measured at fair value on a recurring basis | Level 2 | Preferred stock
Assets:
Available-for-sale securities, fair value	18,898	19,062
Trading securities	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	65,516	65,786
Other long-term investments	149,734	115,516
Short-term investments	0	0
Total investments	1,433,966	1,498,010
Cash	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	1,433,966	1,498,010
Liabilities:
Annuity account balances	85,028	87,616
Other liabilities	561,296	405,803
Total liabilities measured at fair value on a recurring basis	646,324	493,419
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Available-for-sale securities, fair value	1,177,702	1,226,653
Trading securities	41,014	90,055
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	0	3
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	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, fair value	553,149	562,604
Trading securities	35,490	84,563
Measured at fair value on a recurring basis | Level 3 | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	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, fair value	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, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Corporate securities
Assets:
Available-for-sale securities, fair value	624,553	664,046
Trading securities	5,524	5,492
Measured at fair value on a recurring basis | Level 3 | Preferred stock
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	$ 0	$ 0