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	51,751,205	41,176,052
Equity securities	680,232	754,360
Other long-term investments	536,727	605,075
Short-term investments	709,703	615,210
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	49,255,324
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:
Total investments	515,379	504,228
Level 3 | Corporate securities
Assets:
Total investments	637,479	617,770
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	51,751,205	41,176,052
Equity securities	680,232	754,360
Other long-term investments	536,727	605,075
Short-term investments	709,703	615,210
Total investments	53,677,867	43,150,697
Cash	190,732	252,310
Other assets	29,376	28,771
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	68,350,136	58,423,051
Liabilities:
Annuity account balances	80,098	83,472
Other liabilities	759,264	1,007,572
Total liabilities measured at fair value on a recurring basis	839,362	1,091,044
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	49,255,324	38,511,863
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	3,243,980	2,327,212
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	2,227,402	1,889,822
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	401,730	287,282
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	36,743,236	29,702,010
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	614,113	688,214
Other long-term investments	63,220	51,102
Short-term investments	644,486	482,461
Total investments	2,396,784	2,277,533
Cash	190,732	252,310
Other assets	29,376	28,771
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	17,069,053	17,549,887
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	49,416,615	38,948,366
Equity securities	36	36
Other long-term investments	316,833	417,969
Short-term investments	65,217	132,749
Total investments	49,798,701	39,499,120
Cash	0	0
Other assets	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	49,798,701	39,499,120
Liabilities:
Annuity account balances	0	0
Other liabilities	181,176	240,927
Total liabilities measured at fair value on a recurring basis	181,176	240,927
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	46,984,890	36,349,351
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	3,222,200	2,327,212
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	2,180,175	1,889,822
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	401,730	287,282
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	36,098,425	29,075,109
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	66,083	66,110
Other long-term investments	156,674	136,004
Short-term investments	0	0
Total investments	1,482,382	1,374,044
Cash	0	0
Other assets	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	1,482,382	1,374,044
Liabilities:
Annuity account balances	80,098	83,472
Other liabilities	572,916	760,890
Total liabilities measured at fair value on a recurring basis	653,014	844,362
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