FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Fixed maturity securities - trading	$ 2,550,330	$ 2,725,694
Total fixed maturity securities	51,552,705	40,971,486
Equity securities	642,259	715,498
Other long-term investments	550,647	629,537
Short-term investments	597,614	527,144
Assets related to separate accounts
Variable annuity	13,414,407	13,956,071
Variable universal life	1,037,754	1,035,202
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	49,056,824
Fixed maturity securities - trading	2,495,881	2,664,189
Residential mortgage-backed securities
Assets:
Fixed maturity securities - trading	259,755	259,694
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - trading	141,469	146,804
Other asset-backed securities
Assets:
Fixed maturity securities - trading	114,507	138,097
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - trading	314,722	326,925
Other government-related securities
Assets:
Fixed maturity securities - trading	40,274	63,925
Corporate securities
Assets:
Fixed maturity securities - trading	1,585,290	1,698,183
Level 3 | Other asset-backed securities
Assets related to separate accounts
Total assets measured at fair value on a recurring basis	515,379	504,228
Level 3 | Corporate securities
Assets related to separate accounts
Total assets measured at fair value on a recurring basis	637,479	617,770
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	51,552,706	40,971,486
Equity securities	642,260	715,498
Other long-term investments	550,647	629,537
Short-term investments	597,614	527,144
Total investments	53,343,227	42,843,665
Cash	164,232	178,855
Assets related to separate accounts
Variable annuity	13,414,407	13,956,071
Variable universal life	1,037,754	1,035,202
Total assets measured at fair value on a recurring basis	67,959,620	58,013,793
Liabilities:
Annuity account balances	80,098	83,472
Other liabilities	711,044	905,973
Total liabilities measured at fair value on a recurring basis	791,142	989,445
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	49,056,825	38,307,297
Fixed maturity securities - trading	2,495,881	2,664,189
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	3,235,395	2,318,493
Fixed maturity securities - trading	259,755	259,694
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,198,883	1,860,488
Fixed maturity securities - trading	141,469	146,804
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,423,561	1,249,549
Fixed maturity securities - trading	114,507	138,097
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,492,036	1,223,252
Fixed maturity securities - trading	36,575	27,234
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	3,633,803	1,741,645
Fixed maturity securities - trading	314,722	326,925
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	399,772	285,233
Fixed maturity securities - trading	40,274	63,925
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	36,583,799	29,537,546
Fixed maturity securities - trading	1,585,290	1,698,183
Measured at fair value on a recurring basis | Redeemable preferred stock
Assets:
Fixed maturity securities - available-for-sale	89,576	91,091
Fixed maturity securities - trading	3,289	3,327
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,074,965	1,055,756
Equity securities	576,707	649,981
Other long-term investments	63,220	51,102
Short-term investments	532,397	394,394
Total investments	2,247,289	2,151,233
Cash	164,232	178,855
Assets related to separate accounts
Variable annuity	13,414,407	13,956,071
Variable universal life	1,037,754	1,035,202
Total assets measured at fair value on a recurring basis	16,863,682	17,321,361
Liabilities:
Annuity account balances	0	0
Other liabilities	5,172	5,755
Total liabilities measured at fair value on a recurring basis	5,172	5,755
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,040,915	1,031,246
Fixed maturity securities - trading	34,050	24,510
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	969,623	958,775
Fixed maturity securities - trading	30,761	21,183
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 stock
Assets:
Fixed maturity securities - available-for-sale	71,292	72,471
Fixed maturity securities - trading	3,289	3,327
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	49,218,116	38,743,800
Equity securities	0	0
Other long-term investments	316,833	417,969
Short-term investments	65,217	132,750
Total investments	49,600,166	39,294,519
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	49,600,166	39,294,519
Liabilities:
Annuity account balances	0	0
Other liabilities	248,864	302,656
Total liabilities measured at fair value on a recurring basis	248,864	302,656
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	46,786,391	36,144,785
Fixed maturity securities - trading	2,431,725	2,599,015
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	3,213,615	2,318,493
Fixed maturity securities - trading	259,755	259,694
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,151,656	1,860,488
Fixed maturity securities - trading	141,469	146,804
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	907,860	745,184
Fixed maturity securities - trading	89,655	102,875
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	522,413	264,477
Fixed maturity securities - trading	5,814	6,051
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	3,633,803	1,741,645
Fixed maturity securities - trading	314,722	326,925
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	399,772	285,233
Fixed maturity securities - trading	40,274	63,925
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	35,938,988	28,910,645
Fixed maturity securities - trading	1,580,036	1,692,741
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stock
Assets:
Fixed maturity securities - available-for-sale	18,284	18,620
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,259,625	1,171,930
Equity securities	65,553	65,517
Other long-term investments	170,594	160,466
Short-term investments	0	0
Total investments	1,495,772	1,397,913
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	1,495,772	1,397,913
Liabilities:
Annuity account balances	80,098	83,472
Other liabilities	457,008	597,562
Total liabilities measured at fair value on a recurring basis	537,106	681,034
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,229,519	1,131,266
Fixed maturity securities - trading	30,106	40,664
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	21,780	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	47,227	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	515,701	504,365
Fixed maturity securities - trading	24,852	35,222
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	644,811	626,901
Fixed maturity securities - trading	5,254	5,442
Measured at fair value on a recurring basis | Level 3 | Redeemable preferred stock
Assets:
Fixed maturity securities - available-for-sale	0	0
Fixed maturity securities - trading	$ 0	$ 0