FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Fixed maturity securities—available-for-sale	$ 50,264,864	$ 39,819,928
Fixed maturity securities—trading	2,457,010	2,725,694
Total fixed maturity securities	51,904,699	41,176,052
Equity securities	595,884	754,360
Other long-term investments	375,829	605,075
Short-term investments	807,283	615,210
Assets related to separate accounts
Variable annuity	12,288,919	13,956,071
Variable universal life	937,732	1,035,202
Fixed maturities
Assets:
Fixed maturity securities—available-for-sale	49,488,507
Fixed maturity securities—trading	2,416,192	2,664,189
Residential mortgage-backed securities
Assets:
Fixed maturity securities—trading	241,836	259,694
Commercial mortgage-backed securities
Assets:
Fixed maturity securities—trading	188,925	146,804
Other asset-backed securities
Assets:
Fixed maturity securities—trading	159,907	138,097
States, municipals, and political subdivisions
Assets:
Fixed maturity securities—trading	286,413	326,925
Other government-related securities
Assets:
Fixed maturity securities—trading	44,207	63,925
Corporate securities
Assets:
Fixed maturity securities—trading	1,423,833	1,698,183
Level 3 | Other asset-backed securities
Assets:
Total investments	421,458	504,228
Level 3 | Corporate securities
Assets:
Total investments	631,068	617,770
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	51,904,699	41,176,052
Equity securities	595,884	754,360
Other long-term investments	375,829	605,075
Short-term investments	807,283	615,210
Total investments	53,683,695	43,150,697
Cash	173,714	252,310
Other assets	29,257	28,771
Assets related to separate accounts
Variable annuity	12,288,919	13,956,071
Variable universal life	937,732	1,035,202
Total assets measured at fair value on a recurring basis	67,113,317	58,423,051
Liabilities:
Annuity account balances	76,119	83,472
Other liabilities	755,461	1,007,572
Total liabilities measured at fair value on a recurring basis	831,580	1,091,044
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities—available-for-sale	49,488,507	38,511,863
Fixed maturity securities—trading	2,416,192	2,664,189
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities—available-for-sale	3,611,590	2,327,212
Fixed maturity securities—trading	241,836	259,694
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities—available-for-sale	2,295,084	1,889,822
Fixed maturity securities—trading	188,925	146,804
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities—available-for-sale	1,391,893	1,249,549
Fixed maturity securities—trading	159,907	138,097
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities—available-for-sale	1,639,505	1,223,252
Fixed maturity securities—trading	59,794	27,234
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities—available-for-sale	3,588,841	1,741,645
Fixed maturity securities—trading	286,413	326,925
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities—available-for-sale	515,964	287,282
Fixed maturity securities—trading	44,207	63,925
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities—available-for-sale	36,362,828	29,702,010
Fixed maturity securities—trading	1,423,833	1,698,183
Measured at fair value on a recurring basis | Redeemable preferred stock
Assets:
Fixed maturity securities—available-for-sale	82,802	91,091
Fixed maturity securities—trading	11,277	3,327
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,114,751	1,055,756
Equity securities	531,523	688,214
Other long-term investments	83,047	51,102
Short-term investments	730,067	482,461
Total investments	2,459,388	2,277,533
Cash	173,714	252,310
Other assets	29,257	28,771
Assets related to separate accounts
Variable annuity	12,288,919	13,956,071
Variable universal life	937,732	1,035,202
Total assets measured at fair value on a recurring basis	15,889,010	17,549,887
Liabilities:
Annuity account balances	0	0
Other liabilities	56,018	5,755
Total liabilities measured at fair value on a recurring basis	56,018	5,755
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities—available-for-sale	1,076,021	1,031,246
Fixed maturity securities—trading	38,730	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	1,010,485	958,775
Fixed maturity securities—trading	27,453	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	65,536	72,471
Fixed maturity securities—trading	11,277	3,327
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	49,697,732	38,948,366
Equity securities	36	36
Other long-term investments	180,438	417,969
Short-term investments	77,216	132,749
Total investments	49,955,422	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	49,955,422	39,499,120
Liabilities:
Annuity account balances	0	0
Other liabilities	69,501	240,927
Total liabilities measured at fair value on a recurring basis	69,501	240,927
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities—available-for-sale	47,352,568	36,349,351
Fixed maturity securities—trading	2,345,164	2,599,015
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities—available-for-sale	3,611,590	2,327,212
Fixed maturity securities—trading	241,836	259,694
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities—available-for-sale	2,295,084	1,889,822
Fixed maturity securities—trading	188,925	146,804
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities—available-for-sale	970,251	745,184
Fixed maturity securities—trading	133,851	102,875
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities—available-for-sale	629,020	264,477
Fixed maturity securities—trading	32,341	6,051
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities—available-for-sale	3,588,841	1,741,645
Fixed maturity securities—trading	286,413	326,925
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities—available-for-sale	515,964	287,282
Fixed maturity securities—trading	44,207	63,925
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities—available-for-sale	35,724,552	29,075,109
Fixed maturity securities—trading	1,417,591	1,692,741
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stock
Assets:
Fixed maturity securities—available-for-sale	17,266	18,620
Fixed maturity securities—trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,092,216	1,171,930
Equity securities	64,325	66,110
Other long-term investments	112,344	136,004
Short-term investments	0	0
Total investments	1,268,885	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	1,268,885	1,374,044
Liabilities:
Annuity account balances	76,119	83,472
Other liabilities	629,942	760,890
Total liabilities measured at fair value on a recurring basis	706,061	844,362
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities—available-for-sale	1,059,918	1,131,266
Fixed maturity securities—trading	32,298	40,664
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	0	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	421,642	504,365
Fixed maturity securities—trading	26,056	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	638,276	626,901
Fixed maturity securities—trading	6,242	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