FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets:
Fixed maturity securities - available-for-sale	$ 67,160,250	$ 64,746,076
Fixed maturity securities - trading	2,748,839	2,625,436
Total fixed maturity securities	68,780,104	66,043,992
Equity securities	513,181	553,720
Other long-term investments	1,172,964	995,493
Short-term investments	1,113,261	1,320,864
Assets related to separate accounts
Variable annuity	11,394,911	12,730,090
Variable universal life	1,087,327	1,135,666
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	66,122,281	63,516,425
Fixed maturity securities - trading	2,657,823	2,527,567
Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	7,014,129	5,931,341
Fixed maturity securities - trading	192,944	209,521
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,541,482	2,639,668
Fixed maturity securities - trading	189,995	201,284
Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,702,024	1,781,235
Fixed maturity securities - trading	144,248	143,361
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,562,366	4,638,850
Fixed maturity securities - trading	289,477	293,791
Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	626,170	597,169
Fixed maturity securities - trading	29,210	28,775
Corporate securities
Assets:
Fixed maturity securities - available-for-sale	48,436,519	46,809,101
Fixed maturity securities - trading	1,757,339	1,590,936
Level 3 | Commercial mortgage-backed securities
Assets:
Total investments	9,971	10,029
Level 3 | Other asset-backed securities
Assets:
Total investments	430,964	421,219
Level 3 | Corporate securities
Assets:
Total investments	1,365,963	1,373,714
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	68,780,104	66,043,992
Equity securities	513,181	553,720
Other long-term investments	1,172,964	995,493
Short-term investments	1,113,261	1,320,864
Total investments	71,579,510	68,914,069
Cash	445,447	171,752
Assets related to separate accounts
Variable annuity	11,394,911	12,730,090
Variable universal life	1,087,327	1,135,666
Total assets measured at fair value on a recurring basis	84,507,195	82,951,577
Liabilities:
Annuity account balances	68,064	69,728
Other liabilities	2,155,762	1,547,178
Total liabilities measured at fair value on a recurring basis	2,223,826	1,616,906
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	66,122,281	63,516,425
Fixed maturity securities - trading	2,657,823	2,527,567
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	7,014,129	5,931,341
Fixed maturity securities - trading	192,944	209,521
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,541,482	2,639,668
Fixed maturity securities - trading	189,995	201,284
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,702,025	1,781,235
Fixed maturity securities - trading	144,248	143,361
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,173,918	1,032,396
Fixed maturity securities - trading	42,677	47,067
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,562,366	4,638,850
Fixed maturity securities - trading	289,477	293,791
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	626,170	597,169
Fixed maturity securities - trading	29,210	28,775
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	48,436,518	46,809,101
Fixed maturity securities - trading	1,757,339	1,590,936
Measured at fair value on a recurring basis | Redeemable preferred stocks
Assets:
Fixed maturity securities - available-for-sale	65,673	86,665
Fixed maturity securities - trading	11,933	12,832
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	773,131	770,199
Equity securities	434,568	480,750
Other long-term investments	81,357	52,225
Short-term investments	1,010,740	1,255,384
Total investments	2,299,796	2,558,558
Cash	445,447	171,752
Assets related to separate accounts
Variable annuity	11,394,911	12,730,090
Variable universal life	1,087,327	1,135,666
Total assets measured at fair value on a recurring basis	15,227,481	16,596,066
Liabilities:
Annuity account balances	0	0
Other liabilities	6,417	19,561
Total liabilities measured at fair value on a recurring basis	6,417	19,561
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	735,165	732,557
Fixed maturity securities - trading	37,966	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	669,492	662,581
Fixed maturity securities - trading	26,033	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	65,673	69,976
Fixed maturity securities - trading	11,933	12,832
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	66,127,295	63,392,053
Equity securities	0	0
Other long-term investments	815,700	733,425
Short-term investments	102,521	65,480
Total investments	67,045,516	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	67,045,516	64,190,958
Liabilities:
Annuity account balances	0	0
Other liabilities	485,094	509,645
Total liabilities measured at fair value on a recurring basis	485,094	509,645
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	63,580,218	60,978,906
Fixed maturity securities - trading	2,547,077	2,413,147
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	7,014,129	5,931,341
Fixed maturity securities - trading	192,944	209,521
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,531,511	2,629,639
Fixed maturity securities - trading	189,995	201,284
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,271,061	1,360,016
Fixed maturity securities - trading	83,626	77,954
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	504,426	369,815
Fixed maturity securities - trading	16,644	22,257
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	4,562,366	4,638,850
Fixed maturity securities - trading	289,477	293,791
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	626,170	597,169
Fixed maturity securities - trading	29,210	28,775
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	47,070,555	45,435,387
Fixed maturity securities - trading	1,745,181	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	1,879,678	1,881,740
Equity securities	78,613	72,970
Other long-term investments	275,907	209,843
Short-term investments	0	0
Total investments	2,234,198	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	2,234,198	2,164,553
Liabilities:
Annuity account balances	68,064	69,728
Other liabilities	1,664,251	1,017,972
Total liabilities measured at fair value on a recurring basis	1,732,315	1,087,700
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,806,898	1,804,962
Fixed maturity securities - trading	72,780	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	9,971	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	430,964	421,219
Fixed maturity securities - trading	60,622	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	1,365,963	1,373,714
Fixed maturity securities - trading	12,158	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