FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Fixed maturity securities - available-for-sale	$ 50,385,896	$ 39,819,928
Fixed maturity securities - trading	52,381,464	41,176,052
Equity securities	662,880	754,360
Other long-term investments	636,586	605,075
Short-term investments	532,053	615,210
Assets related to separate accounts
Variable annuity	13,555,469	13,956,071
Variable universal life	1,077,842	1,035,202
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	49,909,708
Level 3 | Other asset-backed securities
Assets:
Total investments	502,020	504,228
Level 3 | Corporate securities
Assets:
Total investments	621,392	617,770
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	52,381,464	41,176,052
Equity securities	662,880	754,360
Other long-term investments	636,586	605,075
Short-term investments	532,053	615,210
Total investments	54,212,983	43,150,697
Cash	194,460	252,310
Other assets	31,743	28,771
Assets related to separate accounts
Variable annuity	13,555,469	13,956,071
Variable universal life	1,077,842	1,035,202
Total assets measured at fair value on a recurring basis	69,072,497	58,423,051
Liabilities:
Annuity account balances	78,463	83,472
Other liabilities	835,447	1,007,572
Total liabilities measured at fair value on a recurring basis	913,910	1,091,044
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	49,909,708	38,511,863
Fixed maturity securities - trading	2,471,756	2,664,189
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	3,415,105	2,327,212
Fixed maturity securities - trading	254,037	259,694
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,262,771	1,889,822
Fixed maturity securities - trading	135,403	146,804
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,454,135	1,249,549
Fixed maturity securities - trading	111,770	138,097
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,580,979	1,223,252
Fixed maturity securities - trading	58,081	27,234
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	3,558,073	1,741,645
Fixed maturity securities - trading	296,618	326,925
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	529,581	287,282
Fixed maturity securities - trading	43,030	63,925
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	37,021,925	29,702,010
Fixed maturity securities - trading	1,569,652	1,698,183
Measured at fair value on a recurring basis | Redeemable preferred stock
Assets:
Fixed maturity securities - available-for-sale	87,139	91,091
Fixed maturity securities - trading	3,165	3,327
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,046,253	1,055,756
Equity securities	598,576	688,214
Other long-term investments	52,728	51,102
Short-term investments	434,734	482,461
Total investments	2,132,291	2,277,533
Cash	194,460	252,310
Other assets	31,743	28,771
Assets related to separate accounts
Variable annuity	13,555,469	13,956,071
Variable universal life	1,077,842	1,035,202
Total assets measured at fair value on a recurring basis	16,991,805	17,549,887
Liabilities:
Annuity account balances	0	0
Other liabilities	8,243	5,755
Total liabilities measured at fair value on a recurring basis	8,243	5,755
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,016,229	1,031,246
Fixed maturity securities - trading	30,024	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	946,679	958,775
Fixed maturity securities - trading	26,859	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	69,550	72,471
Fixed maturity securities - trading	3,165	3,327
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	50,151,964	38,948,366
Equity securities	36	36
Other long-term investments	402,312	417,969
Short-term investments	97,319	132,749
Total investments	50,651,631	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	50,651,631	39,499,120
Liabilities:
Annuity account balances	0	0
Other liabilities	272,724	240,927
Total liabilities measured at fair value on a recurring basis	272,724	240,927
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	47,743,467	36,349,351
Fixed maturity securities - trading	2,408,497	2,599,015
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	3,415,105	2,327,212
Fixed maturity securities - trading	254,037	259,694
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,243,304	1,889,822
Fixed maturity securities - trading	135,403	146,804
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	951,874	745,184
Fixed maturity securities - trading	83,758	102,875
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	634,300	264,477
Fixed maturity securities - trading	31,222	6,051
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	3,558,073	1,741,645
Fixed maturity securities - trading	296,618	326,925
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	529,581	287,282
Fixed maturity securities - trading	43,030	63,925
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	36,393,641	29,075,109
Fixed maturity securities - trading	1,564,429	1,692,741
Measured at fair value on a recurring basis | Level 2 | Redeemable preferred stock
Assets:
Fixed maturity securities - available-for-sale	17,589	18,620
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,183,247	1,171,930
Equity securities	64,268	66,110
Other long-term investments	181,546	136,004
Short-term investments	0	0
Total investments	1,429,061	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	1,429,061	1,374,044
Liabilities:
Annuity account balances	78,463	83,472
Other liabilities	554,480	760,890
Total liabilities measured at fair value on a recurring basis	632,943	844,362
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,150,012	1,131,266
Fixed maturity securities - trading	33,235	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	19,467	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	502,261	504,365
Fixed maturity securities - trading	28,012	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	628,284	626,901
Fixed maturity securities - trading	5,223	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