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	$ 64,872,331	$ 49,898,408
Fixed maturity securities - trading	2,642,144	2,457,010
Total fixed maturity securities	66,463,386	51,679,226
Equity securities	581,421	557,708
Other long-term investments	823,514	414,827
Short-term investments	1,044,442	666,301
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	63,926,872	49,263,033
Fixed maturity securities - trading	2,536,514	2,416,192
Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	5,340,663	3,602,991
Fixed maturity securities - trading	202,963	241,836
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,714,466	2,266,387
Fixed maturity securities - trading	208,935	188,925
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
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,834,657	3,588,841
Fixed maturity securities - trading	300,332	286,413
Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	603,236	514,036
Fixed maturity securities - trading	26,792	44,207
Corporate securities
Assets:
Fixed maturity securities - available-for-sale	47,285,605	36,201,578
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	66,463,387	51,679,225
Equity securities	581,421	557,708
Other long-term investments	823,514	414,827
Short-term investments	1,044,442	666,301
Total investments	68,912,764	53,318,061
Cash	223,300	151,400
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	82,745,275	66,696,112
Liabilities:
Annuity account balances	71,820	76,119
Other liabilities	1,424,955	658,788
Total liabilities measured at fair value on a recurring basis	1,496,775	734,907
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	63,926,873	49,263,033
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	5,340,664	3,602,991
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	2,714,466	2,266,387
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	1,614,505
Fixed maturity securities - trading	48,112	59,794
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 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	603,236	514,036
Fixed maturity securities - trading	26,792	44,207
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	47,285,605	36,201,578
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	1,089,751
Equity securities	512,937	494,287
Other long-term investments	62,772	83,047
Short-term investments	963,836	589,084
Total investments	2,450,054	2,256,169
Cash	223,300	151,400
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	16,282,565	15,634,220
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	1,051,021
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	985,485
Fixed maturity securities - trading	25,756	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	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	63,733,506	49,497,258
Equity securities	0	0
Other long-term investments	567,702	180,438
Short-term investments	80,606	77,217
Total investments	64,381,814	49,754,913
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	64,381,814	49,754,913
Liabilities:
Annuity account balances	0	0
Other liabilities	345,085	164,643
Total liabilities measured at fair value on a recurring basis	345,085	164,643
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	61,303,506	47,152,094
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	5,340,664	3,602,991
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	2,704,831	2,266,387
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 | 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 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	603,236	514,036
Fixed maturity securities - trading	26,792	44,207
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	45,960,493	35,563,302
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	68,484	63,421
Other long-term investments	193,040	151,342
Short-term investments	0	0
Total investments	2,080,896	1,306,979
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	2,080,896	1,306,979
Liabilities:
Annuity account balances	71,820	76,119
Other liabilities	1,067,291	438,127
Total liabilities measured at fair value on a recurring basis	1,139,111	514,246
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 | 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	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