FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Fixed maturity securities - available-for-sale	$ 52,066,902	$ 50,264,864
Fixed maturity securities - trading	2,537,299	2,457,010
Total fixed maturity securities	53,777,352	51,904,699
Equity securities	619,440	595,884
Other long-term investments	537,775	375,829
Short-term investments	817,642	807,283
Assets related to separate accounts:
Variable annuity	12,737,450	12,288,919
Variable universal life	1,041,397	937,732
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	51,283,891
Fixed maturity securities - trading	2,493,461	2,416,192
Residential mortgage-backed securities
Assets:
Fixed maturity securities - trading	213,259	241,836
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - trading	209,482	188,925
Other asset-backed securities
Assets:
Fixed maturity securities - trading	149,541	159,907
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - trading	292,796	286,413
Other government-related securities
Assets:
Fixed maturity securities - trading	23,640	44,207
Corporate securities
Assets:
Fixed maturity securities - trading	1,533,256	1,423,833
Level 3 | Other asset-backed securities
Assets:
Total investments	419,915	421,458
Level 3 | Corporate securities
Assets:
Total investments	648,608	631,068
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	53,777,352	51,904,699
Equity securities	619,440	595,884
Other long-term investments	537,775	375,829
Short-term investments	817,642	807,283
Total investments	55,752,209	53,683,695
Cash	278,630	173,714
Other assets	32,510	29,257
Assets related to separate accounts:
Variable annuity	12,737,450	12,288,919
Variable universal life	1,041,397	937,732
Total assets measured at fair value on a recurring basis	69,842,196	67,113,317
Liabilities:
Annuity account balances	74,613	76,119
Other liabilities	972,965	755,461
Total liabilities measured at fair value on a recurring basis	1,047,578	831,580
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	51,283,891	49,488,507
Fixed maturity securities - trading	2,493,461	2,416,192
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	3,908,288	3,611,590
Fixed maturity securities - trading	213,259	241,836
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,353,344	2,295,084
Fixed maturity securities - trading	209,482	188,925
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,362,611	1,391,893
Fixed maturity securities - trading	149,541	159,907
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,411,106	1,639,505
Fixed maturity securities - trading	59,627	59,794
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	3,691,664	3,588,841
Fixed maturity securities - trading	292,796	286,413
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	524,161	515,964
Fixed maturity securities - trading	23,640	44,207
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	37,948,894	36,362,828
Fixed maturity securities - trading	1,533,256	1,423,833
Measured at fair value on a recurring basis | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	83,823	82,802
Fixed maturity securities - trading	11,860	11,277
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,016,527	1,114,751
Equity securities	555,009	531,523
Other long-term investments	68,379	83,047
Short-term investments	722,906	730,067
Total investments	2,362,821	2,459,388
Cash	278,630	173,714
Other assets	32,510	29,257
Assets related to separate accounts:
Variable annuity	12,737,450	12,288,919
Variable universal life	1,041,397	937,732
Total assets measured at fair value on a recurring basis	16,452,808	15,889,010
Liabilities:
Annuity account balances	0	0
Other liabilities	18,581	56,018
Total liabilities measured at fair value on a recurring basis	18,581	56,018
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	977,787	1,076,021
Fixed maturity securities - trading	38,740	38,730
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	911,137	1,010,485
Fixed maturity securities - trading	26,880	27,453
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 | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	66,650	65,536
Fixed maturity securities - trading	11,860	11,277
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	51,620,391	49,697,732
Equity securities	37	36
Other long-term investments	359,864	180,438
Short-term investments	94,736	77,216
Total investments	52,075,028	49,955,422
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	52,075,028	49,955,422
Liabilities:
Annuity account balances	0	0
Other liabilities	156,493	69,501
Total liabilities measured at fair value on a recurring basis	156,493	69,501
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	49,237,405	47,352,568
Fixed maturity securities - trading	2,382,986	2,345,164
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	3,908,288	3,611,590
Fixed maturity securities - trading	213,259	241,836
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,353,344	2,295,084
Fixed maturity securities - trading	209,482	188,925
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	942,520	970,251
Fixed maturity securities - trading	83,057	133,851
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	499,969	629,020
Fixed maturity securities - trading	32,747	32,341
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	3,691,664	3,588,841
Fixed maturity securities - trading	292,796	286,413
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	524,161	515,964
Fixed maturity securities - trading	23,640	44,207
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	37,300,286	35,724,552
Fixed maturity securities - trading	1,528,005	1,417,591
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	17,173	17,266
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,140,434	1,092,216
Equity securities	64,394	64,325
Other long-term investments	109,532	112,344
Short-term investments	0	0
Total investments	1,314,360	1,268,885
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	1,314,360	1,268,885
Liabilities:
Annuity account balances	74,613	76,119
Other liabilities	797,891	629,942
Total liabilities measured at fair value on a recurring basis	872,504	706,061
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,068,699	1,059,918
Fixed maturity securities - trading	71,735	32,298
Measured at fair value on a recurring basis | Level 3 | 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 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	420,091	421,642
Fixed maturity securities - trading	66,484	26,056
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	648,608	638,276
Fixed maturity securities - trading	5,251	6,242
Measured at fair value on a recurring basis | Level 3 | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	$ 0	$ 0