FAIR VALUE OF FINANCIAL INSTRUMENTS - Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Fixed maturity securities - AFS	$ 69,763,824	$ 64,746,076
Fixed maturity securities - trading	2,958,145	2,625,436
Total fixed maturity securities	72,239,927	66,043,992
Equity securities	666,977	553,720
Other long-term investments	1,559,405	995,493
Short-term investments	462,415	1,320,864
Assets related to separate accounts
Variable annuity	12,377,571	12,730,090
Variable universal life	1,286,570	1,135,666
Residential mortgage-backed securities
Assets:
Fixed maturity securities - AFS	6,658,591	5,931,341
Fixed maturity securities - trading	209,240	209,521
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - AFS	2,506,784	2,639,668
Fixed maturity securities - trading	214,107	201,284
Other asset-backed securities
Assets:
Fixed maturity securities - AFS	1,578,456	1,781,235
Fixed maturity securities - trading	162,641	143,361
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - AFS	4,418,995	4,638,850
Fixed maturity securities - trading	281,803	293,791
Other government-related securities
Assets:
Fixed maturity securities - AFS	715,993	597,169
Fixed maturity securities - trading	30,060	28,775
Corporate securities
Assets:
Fixed maturity securities - AFS	51,938,753	46,809,101
Fixed maturity securities - trading	1,860,273	1,590,936
Total fixed maturity securities - available-for-sale
Assets:
Fixed maturity securities - AFS	69,377,559	63,516,425
Fixed maturity securities - trading	2,862,368	2,527,567
Level 2 | Corporate securities
Assets:
Total investments	58,400,000
Level 3 | Commercial mortgage-backed securities
Assets:
Total investments	32,425	10,029
Level 3 | Other asset-backed securities
Assets:
Total investments	434,676	421,219
Level 3 | Corporate securities
Assets:
Total investments	1,431,818	1,373,714
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	72,239,927	66,043,992
Equity securities	666,977	553,720
Other long-term investments	1,559,405	995,493
Short-term investments	462,415	1,320,864
Total investments	74,928,724	68,914,069
Cash	581,390	171,752
Assets related to separate accounts
Variable annuity	12,377,571	12,730,090
Variable universal life	1,286,570	1,135,666
Total assets measured at fair value on a recurring basis	89,174,255	82,951,577
Liabilities:
Annuity account balances	66,973	69,728
Other liabilities	2,681,451	1,547,178
Total liabilities measured at fair value on a recurring basis	2,748,424	1,616,906
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - AFS	6,658,591	5,931,341
Fixed maturity securities - trading	209,240	209,521
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - AFS	2,506,784	2,639,668
Fixed maturity securities - trading	214,107	201,284
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - AFS	1,578,456	1,781,235
Fixed maturity securities - trading	162,641	143,361
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - AFS	1,366,008	1,032,396
Fixed maturity securities - trading	91,288	47,067
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - AFS	4,418,995	4,638,850
Fixed maturity securities - trading	281,803	293,791
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - AFS	715,993	597,169
Fixed maturity securities - trading	30,060	28,775
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - AFS	51,938,753	46,809,101
Fixed maturity securities - trading	1,860,273	1,590,936
Measured at fair value on a recurring basis | Redeemable preferred stocks
Assets:
Fixed maturity securities - AFS	193,979	86,665
Fixed maturity securities - trading	12,956	12,832
Measured at fair value on a recurring basis | Total fixed maturity securities - available-for-sale
Assets:
Fixed maturity securities - AFS	69,377,559	63,516,425
Fixed maturity securities - trading	2,862,368	2,527,567
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,082,526	770,199
Equity securities	565,750	480,750
Other long-term investments	51,736	52,225
Short-term investments	403,745	1,255,384
Total investments	2,103,757	2,558,558
Cash	581,390	171,752
Assets related to separate accounts
Variable annuity	12,377,571	12,730,090
Variable universal life	1,286,570	1,135,666
Total assets measured at fair value on a recurring basis	16,349,288	16,596,066
Liabilities:
Annuity account balances	0	0
Other liabilities	13,819	19,561
Total liabilities measured at fair value on a recurring basis	13,819	19,561
Measured at fair value on a recurring basis | Level 1 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - AFS	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 - AFS	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 - AFS	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 - AFS	865,586	662,581
Fixed maturity securities - trading	79,196	24,810
Measured at fair value on a recurring basis | Level 1 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - AFS	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 - AFS	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 1 | Corporate securities
Assets:
Fixed maturity securities - AFS	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 - AFS	124,788	69,976
Fixed maturity securities - trading	12,956	12,832
Measured at fair value on a recurring basis | Level 1 | Total fixed maturity securities - available-for-sale
Assets:
Fixed maturity securities - AFS	990,374	732,557
Fixed maturity securities - trading	92,152	37,642
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	69,169,322	63,392,053
Equity securities	0	0
Other long-term investments	1,285,674	733,425
Short-term investments	58,670	65,480
Total investments	70,513,666	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	70,513,666	64,190,958
Liabilities:
Annuity account balances	0	0
Other liabilities	923,805	509,645
Total liabilities measured at fair value on a recurring basis	923,805	509,645
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - AFS	6,658,591	5,931,341
Fixed maturity securities - trading	209,240	209,521
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - AFS	2,474,359	2,629,639
Fixed maturity securities - trading	214,107	201,284
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - AFS	1,143,780	1,360,016
Fixed maturity securities - trading	91,241	77,954
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - AFS	500,422	369,815
Fixed maturity securities - trading	12,092	22,257
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - AFS	4,418,995	4,638,850
Fixed maturity securities - trading	281,803	293,791
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - AFS	715,993	597,169
Fixed maturity securities - trading	30,060	28,775
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - AFS	50,506,935	45,435,387
Fixed maturity securities - trading	1,842,513	1,579,565
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stocks
Assets:
Fixed maturity securities - AFS	69,191	16,689
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 2 | Total fixed maturity securities - available-for-sale
Assets:
Fixed maturity securities - AFS	66,488,266	60,978,906
Fixed maturity securities - trading	2,681,056	2,413,147
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,988,079	1,881,740
Equity securities	101,227	72,970
Other long-term investments	221,995	209,843
Short-term investments	0	0
Total investments	2,311,301	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	2,311,301	2,164,553
Liabilities:
Annuity account balances	66,973	69,728
Other liabilities	1,743,827	1,017,972
Total liabilities measured at fair value on a recurring basis	1,810,800	1,087,700
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - AFS	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 - AFS	32,425	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 - AFS	434,676	421,219
Fixed maturity securities - trading	71,400	65,407
Measured at fair value on a recurring basis | Level 3 | U.S. government-related securities
Assets:
Fixed maturity securities - AFS	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 - AFS	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 - AFS	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | Corporate securities
Assets:
Fixed maturity securities - AFS	1,431,818	1,373,714
Fixed maturity securities - trading	17,760	11,371
Measured at fair value on a recurring basis | Level 3 | Redeemable preferred stocks
Assets:
Fixed maturity securities - AFS	0	0
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3 | Total fixed maturity securities - available-for-sale
Assets:
Fixed maturity securities - AFS	1,898,919	1,804,962
Fixed maturity securities - trading	$ 89,160	$ 76,778