FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Fixed maturity securities - available-for-sale	$ 65,643,018	$ 50,264,864
Fixed maturity securities - trading	2,642,144	2,457,010
Total fixed maturity securities	66,680,596	51,904,699
Equity securities	619,341	595,884
Other long-term investments	798,434	375,829
Short-term investments	1,597,919	807,283
Assets related to separate accounts:
Variable annuity	12,542,212	12,288,919
Variable universal life	1,066,999	937,732
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	64,144,082
Fixed maturity securities - trading	2,536,514	2,416,192
Residential mortgage-backed securities
Assets:
Fixed maturity securities - trading	202,963	241,836
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - trading	208,935	188,925
Other asset-backed securities
Assets:
Fixed maturity securities - trading	137,732	159,907
Other government-related securities
Assets:
Fixed maturity securities - trading	26,792	44,207
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - trading	300,332	286,413
Corporate securities
Assets:
Fixed maturity securities - trading	1,599,453	1,423,833
Level 3 | Other asset-backed securities
Assets:
Total investments	415,950	421,458
Level 3 | Corporate securities
Assets:
Total investments	1,325,112	631,068
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	66,680,596	51,904,699
Equity securities	619,341	595,884
Other long-term investments	798,434	375,829
Short-term investments	1,597,919	807,283
Total investments	69,696,290	53,683,695
Cash	293,299	173,714
Other assets	34,621	29,257
Assets related to separate accounts:
Variable annuity	12,542,212	12,288,919
Variable universal life	1,066,999	937,732
Total assets measured at fair value on a recurring basis	83,633,421	67,113,317
Liabilities:
Annuity account balances	71,820	76,119
Other liabilities	1,738,909	755,461
Total liabilities measured at fair value on a recurring basis	1,810,729	831,580
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	64,144,082	49,488,507
Fixed maturity securities - trading	2,536,514	2,416,192
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	5,351,464	3,611,590
Fixed maturity securities - trading	202,963	241,836
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,744,828	2,295,084
Fixed maturity securities - trading	208,935	188,925
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,898,476	1,391,893
Fixed maturity securities - trading	137,732	159,907
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,162,207	1,639,505
Fixed maturity securities - trading	48,112	59,794
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	605,351	515,964
Fixed maturity securities - trading	26,792	44,207
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,834,657	3,588,841
Fixed maturity securities - trading	300,331	286,413
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	47,459,537	36,362,828
Fixed maturity securities - trading	1,599,454	1,423,833
Measured at fair value on a recurring basis | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	87,562	82,802
Fixed maturity securities - trading	12,195	11,277
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	910,509	1,114,751
Equity securities	549,487	531,523
Other long-term investments	62,772	83,047
Short-term investments	1,517,313	730,067
Total investments	3,040,081	2,459,388
Cash	293,299	173,714
Other assets	34,621	29,257
Assets related to separate accounts:
Variable annuity	12,542,212	12,288,919
Variable universal life	1,066,999	937,732
Total assets measured at fair value on a recurring basis	16,977,212	15,889,010
Liabilities:
Annuity account balances	0	0
Other liabilities	12,579	56,018
Total liabilities measured at fair value on a recurring basis	12,579	56,018
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	872,558	1,076,021
Fixed maturity securities - trading	37,951	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	801,931	1,010,485
Fixed maturity securities - trading	25,756	27,453
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 | 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 | 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	70,627	65,536
Fixed maturity securities - trading	12,195	11,277
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	63,950,715	49,697,732
Equity securities	36	36
Other long-term investments	567,702	180,438
Short-term investments	80,606	77,216
Total investments	64,599,059	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	64,599,059	49,955,422
Liabilities:
Annuity account balances	0	0
Other liabilities	251,378	69,501
Total liabilities measured at fair value on a recurring basis	251,378	69,501
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	61,520,715	47,352,568
Fixed maturity securities - trading	2,430,000	2,345,164
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	5,351,464	3,611,590
Fixed maturity securities - trading	202,963	241,836
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,735,193	2,295,084
Fixed maturity securities - trading	208,935	188,925
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,482,414	970,251
Fixed maturity securities - trading	76,205	133,851
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	360,276	629,020
Fixed maturity securities - trading	22,356	32,341
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	605,351	515,964
Fixed maturity securities - trading	26,792	44,207
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,834,657	3,588,841
Fixed maturity securities - trading	300,331	286,413
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	46,134,425	35,724,552
Fixed maturity securities - trading	1,592,418	1,417,591
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	16,935	17,266
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,819,372	1,092,216
Equity securities	69,818	64,325
Other long-term investments	167,960	112,344
Short-term investments	0	0
Total investments	2,057,150	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	2,057,150	1,268,885
Liabilities:
Annuity account balances	71,820	76,119
Other liabilities	1,474,952	629,942
Total liabilities measured at fair value on a recurring basis	1,546,772	706,061
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,750,809	1,059,918
Fixed maturity securities - trading	68,563	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	9,635	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	416,062	421,642
Fixed maturity securities - trading	61,527	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 | 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 | 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 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	1,325,112	638,276
Fixed maturity securities - trading	7,036	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