FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Fixed maturity securities - available-for-sale	$ 63,347,339	$ 50,264,864
Fixed maturity securities - trading	2,592,645	2,457,010
Total fixed maturity securities	64,710,656	51,904,699
Equity securities	629,965	595,884
Other long-term investments	643,796	375,829
Short-term investments	1,220,840	807,283
Assets related to separate accounts:
Variable annuity	12,790,474	12,288,919
Variable universal life	1,073,020	937,732
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	62,208,408
Fixed maturity securities - trading	2,502,248	2,416,192
Residential mortgage-backed securities
Assets:
Fixed maturity securities - trading	203,455	241,836
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - trading	204,445	188,925
Other asset-backed securities
Assets:
Fixed maturity securities - trading	147,997	159,907
Other government-related securities
Assets:
Fixed maturity securities - trading	24,342	44,207
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - trading	293,560	286,413
Corporate securities
Assets:
Fixed maturity securities - trading	1,565,561	1,423,833
Level 3 | Other asset-backed securities
Assets:
Total investments	418,178	421,458
Level 3 | Corporate securities
Assets:
Total investments	1,328,490	631,068
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	64,710,656	51,904,699
Equity securities	629,965	595,884
Other long-term investments	643,796	375,829
Short-term investments	1,220,840	807,283
Total investments	67,205,257	53,683,695
Cash	240,157	173,714
Other assets	34,335	29,257
Assets related to separate accounts:
Variable annuity	12,790,474	12,288,919
Variable universal life	1,073,020	937,732
Total assets measured at fair value on a recurring basis	81,343,243	67,113,317
Liabilities:
Annuity account balances	72,585	76,119
Other liabilities	1,319,193	755,461
Total liabilities measured at fair value on a recurring basis	1,391,778	831,580
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	62,208,408	49,488,507
Fixed maturity securities - trading	2,502,248	2,416,192
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	4,606,248	3,611,590
Fixed maturity securities - trading	203,455	241,836
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,695,861	2,295,084
Fixed maturity securities - trading	204,445	188,925
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,861,598	1,391,893
Fixed maturity securities - trading	147,997	159,907
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,300,557	1,639,505
Fixed maturity securities - trading	51,080	59,794
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	583,149	515,964
Fixed maturity securities - trading	24,342	44,207
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,830,966	3,588,841
Fixed maturity securities - trading	293,560	286,413
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	46,245,117	36,362,828
Fixed maturity securities - trading	1,565,561	1,423,833
Measured at fair value on a recurring basis | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	84,912	82,802
Fixed maturity securities - trading	11,808	11,277
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	989,522	1,114,751
Equity securities	560,302	531,523
Other long-term investments	65,231	83,047
Short-term investments	1,153,978	730,067
Total investments	2,769,033	2,459,388
Cash	240,157	173,714
Other assets	34,335	29,257
Assets related to separate accounts:
Variable annuity	12,790,474	12,288,919
Variable universal life	1,073,020	937,732
Total assets measured at fair value on a recurring basis	16,907,019	15,889,010
Liabilities:
Annuity account balances	0	0
Other liabilities	23,552	56,018
Total liabilities measured at fair value on a recurring basis	23,552	56,018
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	951,785	1,076,021
Fixed maturity securities - trading	37,737	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	884,053	1,010,485
Fixed maturity securities - trading	25,929	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	67,732	65,536
Fixed maturity securities - trading	11,808	11,277
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	61,897,493	49,697,732
Equity securities	36	36
Other long-term investments	467,642	180,438
Short-term investments	66,862	77,216
Total investments	62,432,033	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	62,432,033	49,955,422
Liabilities:
Annuity account balances	0	0
Other liabilities	198,564	69,501
Total liabilities measured at fair value on a recurring basis	198,564	69,501
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	59,500,456	47,352,568
Fixed maturity securities - trading	2,397,037	2,345,164
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	4,606,248	3,611,590
Fixed maturity securities - trading	203,455	241,836
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,686,495	2,295,084
Fixed maturity securities - trading	204,445	188,925
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,443,288	970,251
Fixed maturity securities - trading	85,817	133,851
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	416,504	629,020
Fixed maturity securities - trading	25,151	32,341
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	583,149	515,964
Fixed maturity securities - trading	24,342	44,207
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,830,966	3,588,841
Fixed maturity securities - trading	293,560	286,413
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	44,916,626	35,724,552
Fixed maturity securities - trading	1,560,267	1,417,591
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	17,180	17,266
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,823,641	1,092,216
Equity securities	69,627	64,325
Other long-term investments	110,923	112,344
Short-term investments	0	0
Total investments	2,004,191	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	2,004,191	1,268,885
Liabilities:
Annuity account balances	72,585	76,119
Other liabilities	1,097,077	629,942
Total liabilities measured at fair value on a recurring basis	1,169,662	706,061
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,756,167	1,059,918
Fixed maturity securities - trading	67,474	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	9,366	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	418,310	421,642
Fixed maturity securities - trading	62,180	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	1,328,491	638,276
Fixed maturity securities - trading	5,294	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