FAIR VALUE OF FINANCIAL INSTRUMENTS - Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Trading securities	$ 2,725,694	$ 2,699,591
Total fixed maturity securities	41,176,052	38,183,337
Equity securities	754,360	754,489
Other long-term investments	605,075	542,243
Short-term investments	615,210	332,431
Assets related to separate accounts
Variable annuity	13,956,071	13,244,252
Variable universal life	1,035,202	895,925
Fixed maturities
Assets:
Available-for-sale securities, fair value	38,511,863
Trading securities	2,664,189	2,639,860
Residential mortgage-backed securities
Assets:
Trading securities	259,694	255,027
Commercial mortgage-backed securities
Assets:
Trading securities	146,804	149,683
Other asset-backed securities
Assets:
Trading securities	138,097	200,084
States, municipals, and political subdivisions
Assets:
Trading securities	326,925	316,519
Other government-related securities
Assets:
Trading securities	63,925	63,012
Corporate securities
Assets:
Trading securities	1,698,183	1,624,589
Redeemable preferred stock
Assets:
Trading securities	3,327	3,985
Level 3 | Other asset-backed securities
Assets:
Total investments	504,228	553,308
Level 3 | Corporate securities
Assets:
Total investments	617,770	638,279
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,055,756	1,095,210
Equity securities	688,214	685,443
Other long-term investments	51,102	82,420
Short-term investments	482,461	328,829
Total investments	2,277,533	2,191,902
Cash	252,310	348,182
Other assets	28,771	23,830
Assets related to separate accounts
Variable annuity	13,956,071	13,244,252
Variable universal life	1,035,202	895,925
Total assets measured at fair value on a recurring basis	17,549,887	16,704,091
Liabilities:
Annuity account balances	0	0
Other liabilities	5,755	13,004
Total liabilities measured at fair value on a recurring basis	5,755	13,004
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Available-for-sale securities, fair value	1,031,246	1,068,801
Trading securities	24,510	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	958,775	1,002,020
Trading securities	21,183	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 | Redeemable preferred stock
Assets:
Available-for-sale securities, fair value	72,471	66,781
Trading securities	3,327	3,985
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	38,948,366	35,771,419
Equity securities	36	36
Other long-term investments	417,969	335,498
Short-term investments	132,749	3,602
Total investments	39,499,120	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	39,499,120	36,110,555
Liabilities:
Annuity account balances	0	0
Other liabilities	240,927	163,974
Total liabilities measured at fair value on a recurring basis	240,927	163,974
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Available-for-sale securities, fair value	36,349,351	33,248,023
Trading securities	2,599,015	2,523,396
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	2,327,212	1,898,480
Trading securities	259,694	255,027
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,889,822	1,811,470
Trading securities	146,804	149,683
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	745,184	648,929
Trading securities	102,875	115,521
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	264,477	266,139
Trading securities	6,051	4,537
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,741,645	1,656,503
Trading securities	326,925	316,519
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Available-for-sale securities, fair value	287,282	239,921
Trading securities	63,925	63,012
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Available-for-sale securities, fair value	29,075,109	26,707,519
Trading securities	1,692,741	1,619,097
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stock
Assets:
Available-for-sale securities, fair value	18,620	19,062
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,171,930	1,316,708
Equity securities	66,110	69,010
Other long-term investments	136,004	124,325
Short-term investments	0	0
Total investments	1,374,044	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	1,374,044	1,510,043
Liabilities:
Annuity account balances	83,472	87,616
Other liabilities	760,890	571,843
Total liabilities measured at fair value on a recurring basis	844,362	659,459
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Available-for-sale securities, fair value	1,131,266	1,226,653
Trading securities	40,664	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	504,365	562,604
Trading securities	35,222	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	626,901	664,046
Trading securities	5,442	5,492
Measured at fair value on a recurring basis | Level 3 | Redeemable preferred stock
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Fair Values
Assets:
Total fixed maturity securities	41,176,052	38,183,337
Equity securities	754,360	754,489
Other long-term investments	605,075	542,243
Short-term investments	615,210	332,431
Total investments	43,150,697	39,812,500
Cash	252,310	348,182
Other assets	28,771	23,830
Assets related to separate accounts
Variable annuity	13,956,071	13,244,252
Variable universal life	1,035,202	895,925
Total assets measured at fair value on a recurring basis	58,423,051	54,324,689
Liabilities:
Annuity account balances	83,472	87,616
Other liabilities	1,007,572	748,821
Total liabilities measured at fair value on a recurring basis	1,091,044	836,437
Measured at fair value on a recurring basis | Fair Values | Fixed maturities
Assets:
Available-for-sale securities, fair value	38,511,863	35,543,477
Trading securities	2,664,189	2,639,860
Measured at fair value on a recurring basis | Fair Values | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	2,327,212	1,898,483
Trading securities	259,694	255,027
Measured at fair value on a recurring basis | Fair Values | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,889,822	1,811,470
Trading securities	146,804	149,683
Measured at fair value on a recurring basis | Fair Values | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	1,249,549	1,211,533
Trading securities	138,097	200,084
Measured at fair value on a recurring basis | Fair Values | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	1,223,252	1,268,159
Trading securities	27,234	26,961
Measured at fair value on a recurring basis | Fair Values | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,741,645	1,656,503
Trading securities	326,925	316,519
Measured at fair value on a recurring basis | Fair Values | Other government-related securities
Assets:
Available-for-sale securities, fair value	287,282	239,921
Trading securities	63,925	63,012
Measured at fair value on a recurring basis | Fair Values | Corporate securities
Assets:
Available-for-sale securities, fair value	29,702,010	27,371,565
Trading securities	1,698,183	1,624,589
Measured at fair value on a recurring basis | Fair Values | Redeemable preferred stock
Assets:
Available-for-sale securities, fair value	91,091	85,843
Trading securities	$ 3,327	$ 3,985