FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Fixed maturity securities - available-for-sale	$ 68,473,565	$ 64,746,076
Fixed maturity securities - trading	2,877,449	2,625,436
Total fixed maturity securities	70,095,244	66,043,992
Equity securities	538,385	553,720
Other long-term investments	1,407,774	995,493
Short-term investments	1,239,028	1,320,864
Assets related to separate accounts
Variable annuity	11,487,408	12,730,090
Variable universal life	1,144,474	1,135,666
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	67,311,859	63,516,425
Fixed maturity securities - trading	2,783,385	2,527,567
Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	6,847,273	5,931,341
Fixed maturity securities - trading	207,507	209,521
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,557,002	2,639,668
Fixed maturity securities - trading	208,036	201,284
Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,584,239	1,781,235
Fixed maturity securities - trading	150,290	143,361
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,553,629	4,638,850
Fixed maturity securities - trading	288,085	293,791
Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	638,076	597,169
Fixed maturity securities - trading	29,571	28,775
Corporate securities
Assets:
Fixed maturity securities - available-for-sale	49,984,018	46,809,101
Fixed maturity securities - trading	1,850,471	1,590,936
Level 3 | Commercial mortgage-backed securities
Assets:
Total investments	10,685	10,029
Level 3 | Other asset-backed securities
Assets:
Total investments	434,496	421,219
Level 3 | Corporate securities
Assets:
Total investments	1,357,544	1,373,714
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	70,095,244	66,043,992
Equity securities	538,385	553,720
Other long-term investments	1,407,774	995,493
Short-term investments	1,239,028	1,320,864
Total investments	73,280,431	68,914,069
Cash	381,753	171,752
Assets related to separate accounts
Variable annuity	11,487,408	12,730,090
Variable universal life	1,144,474	1,135,666
Total assets measured at fair value on a recurring basis	86,294,066	82,951,577
Liabilities:
Annuity account balances	67,591	69,728
Other liabilities	2,403,594	1,547,178
Total liabilities measured at fair value on a recurring basis	2,471,185	1,616,906
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	67,311,859	63,516,425
Fixed maturity securities - trading	2,783,385	2,527,567
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	6,847,273	5,931,341
Fixed maturity securities - trading	207,507	209,521
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,557,002	2,639,668
Fixed maturity securities - trading	208,036	201,284
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,584,239	1,781,235
Fixed maturity securities - trading	150,290	143,361
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,080,293	1,032,396
Fixed maturity securities - trading	36,991	47,067
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,553,629	4,638,850
Fixed maturity securities - trading	288,085	293,791
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	638,076	597,169
Fixed maturity securities - trading	29,571	28,775
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	49,984,018	46,809,101
Fixed maturity securities - trading	1,850,471	1,590,936
Measured at fair value on a recurring basis | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	67,329	86,665
Fixed maturity securities - trading	12,434	12,832
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	680,078	770,199
Equity securities	452,042	480,750
Other long-term investments	73,934	52,225
Short-term investments	1,156,203	1,255,384
Total investments	2,362,257	2,558,558
Cash	381,753	171,752
Assets related to separate accounts
Variable annuity	11,487,408	12,730,090
Variable universal life	1,144,474	1,135,666
Total assets measured at fair value on a recurring basis	15,375,892	16,596,066
Liabilities:
Annuity account balances	0	0
Other liabilities	9,480	19,561
Total liabilities measured at fair value on a recurring basis	9,480	19,561
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	645,883	732,557
Fixed maturity securities - trading	34,195	37,642
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	578,554	662,581
Fixed maturity securities - trading	21,761	24,810
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	67,329	69,976
Fixed maturity securities - trading	12,434	12,832
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	67,532,120	63,392,053
Equity securities	0	0
Other long-term investments	1,013,846	733,425
Short-term investments	82,825	65,480
Total investments	68,628,791	64,190,958
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	68,628,791	64,190,958
Liabilities:
Annuity account balances	0	0
Other liabilities	675,209	509,645
Total liabilities measured at fair value on a recurring basis	675,209	509,645
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	64,863,251	60,978,906
Fixed maturity securities - trading	2,668,869	2,413,147
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	6,847,273	5,931,341
Fixed maturity securities - trading	207,507	209,521
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,546,317	2,629,639
Fixed maturity securities - trading	208,036	201,284
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,149,743	1,360,016
Fixed maturity securities - trading	87,848	77,954
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	501,739	369,815
Fixed maturity securities - trading	15,230	22,257
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,553,629	4,638,850
Fixed maturity securities - trading	288,085	293,791
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	638,076	597,169
Fixed maturity securities - trading	29,571	28,775
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	48,626,474	45,435,387
Fixed maturity securities - trading	1,832,592	1,579,565
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	0	16,689
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,883,046	1,881,740
Equity securities	86,343	72,970
Other long-term investments	319,994	209,843
Short-term investments	0	0
Total investments	2,289,383	2,164,553
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	2,289,383	2,164,553
Liabilities:
Annuity account balances	67,591	69,728
Other liabilities	1,718,905	1,017,972
Total liabilities measured at fair value on a recurring basis	1,786,496	1,087,700
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,802,725	1,804,962
Fixed maturity securities - trading	80,321	76,778
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	10,685	10,029
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	434,496	421,219
Fixed maturity securities - trading	62,442	65,407
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	1,357,544	1,373,714
Fixed maturity securities - trading	17,879	11,371
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