FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Fixed maturity securities - trading	$ 2,662,999	$ 2,725,694
Total fixed maturity securities	40,023,550	41,176,052
Equity securities	681,520	754,360
Other long-term investments	586,765	605,075
Short-term investments	441,781	615,210
Assets related to separate accounts
Variable annuity	13,549,068	13,956,071
Variable universal life	1,019,250	1,035,202
Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	37,436,408
Fixed maturity securities - trading	2,587,142	2,664,189
Residential mortgage-backed securities
Assets:
Fixed maturity securities - trading	265,547	259,694
Commercial mortgage-backed securities
Assets:
Fixed maturity securities - trading	146,633	146,804
Other asset-backed securities
Assets:
Fixed maturity securities - trading	129,714	138,097
States, municipals, and political subdivisions
Assets:
Fixed maturity securities - trading	318,142	326,925
Other government-related securities
Assets:
Fixed maturity securities - trading	40,368	63,925
Corporate securities
Assets:
Fixed maturity securities - trading	1,645,899	1,698,183
Level 3 | Other asset-backed securities
Assets:
Total investments	503,672	504,228
Level 3 | Corporate securities
Assets:
Total investments	617,724	617,770
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	40,023,550	41,176,052
Equity securities	681,520	754,360
Other long-term investments	586,765	605,075
Short-term investments	441,781	615,210
Total investments	41,733,616	43,150,697
Cash	307,724	252,310
Other assets	34,463	28,771
Assets related to separate accounts
Variable annuity	13,549,068	13,956,071
Variable universal life	1,019,250	1,035,202
Total assets measured at fair value on a recurring basis	56,644,121	58,423,051
Liabilities:
Annuity account balances	81,399	83,472
Other liabilities	817,705	1,007,572
Total liabilities measured at fair value on a recurring basis	899,104	1,091,044
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	37,436,408	38,511,863
Fixed maturity securities - trading	2,587,142	2,664,189
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,394,370	2,327,212
Fixed maturity securities - trading	265,547	259,694
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,841,506	1,889,822
Fixed maturity securities - trading	146,633	146,804
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,244,652	1,249,549
Fixed maturity securities - trading	129,714	138,097
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	1,327,800	1,223,252
Fixed maturity securities - trading	37,575	27,234
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	1,700,279	1,741,645
Fixed maturity securities - trading	318,142	326,925
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	277,459	287,282
Fixed maturity securities - trading	40,368	63,925
Measured at fair value on a recurring basis | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	28,559,773	29,702,010
Fixed maturity securities - trading	1,645,899	1,698,183
Measured at fair value on a recurring basis | Redeemable Preferred stock
Assets:
Fixed maturity securities - available-for-sale	90,569	91,091
Fixed maturity securities - trading	3,264	3,327
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,148,315	1,055,756
Equity securities	615,423	688,214
Other long-term investments	87,558	51,102
Short-term investments	327,834	482,461
Total investments	2,179,130	2,277,533
Cash	307,724	252,310
Other assets	34,463	28,771
Assets related to separate accounts
Variable annuity	13,549,068	13,956,071
Variable universal life	1,019,250	1,035,202
Total assets measured at fair value on a recurring basis	17,089,635	17,549,887
Liabilities:
Annuity account balances	0	0
Other liabilities	7,397	5,755
Total liabilities measured at fair value on a recurring basis	7,397	5,755
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,113,367	1,031,246
Fixed maturity securities - trading	34,948	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	1,041,123	958,775
Fixed maturity securities - trading	31,684	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	72,244	72,471
Fixed maturity securities - trading	3,264	3,327
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	37,704,755	38,948,366
Equity securities	36	36
Other long-term investments	354,855	417,969
Short-term investments	113,947	132,749
Total investments	38,173,593	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	38,173,593	39,499,120
Liabilities:
Annuity account balances	0	0
Other liabilities	189,206	240,927
Total liabilities measured at fair value on a recurring basis	189,206	240,927
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	35,192,843	36,349,351
Fixed maturity securities - trading	2,511,912	2,599,015
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	2,394,370	2,327,212
Fixed maturity securities - trading	265,547	259,694
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Fixed maturity securities - available-for-sale	1,841,506	1,889,822
Fixed maturity securities - trading	146,633	146,804
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Fixed maturity securities - available-for-sale	740,863	745,184
Fixed maturity securities - trading	94,756	102,875
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Fixed maturity securities - available-for-sale	286,677	264,477
Fixed maturity securities - trading	5,891	6,051
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Fixed maturity securities - available-for-sale	1,700,279	1,741,645
Fixed maturity securities - trading	318,142	326,925
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Fixed maturity securities - available-for-sale	277,459	287,282
Fixed maturity securities - trading	40,368	63,925
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Fixed maturity securities - available-for-sale	27,933,364	29,075,109
Fixed maturity securities - trading	1,640,575	1,692,741
Measured at fair value on a recurring basis | Level 2 | Redeemable Preferred stock
Assets:
Fixed maturity securities - available-for-sale	18,325	18,620
Fixed maturity securities - trading	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,170,480	1,171,930
Equity securities	66,061	66,110
Other long-term investments	144,352	136,004
Short-term investments	0	0
Total investments	1,380,893	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	1,380,893	1,374,044
Liabilities:
Annuity account balances	81,399	83,472
Other liabilities	621,102	760,890
Total liabilities measured at fair value on a recurring basis	702,501	844,362
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Fixed maturity securities - available-for-sale	1,130,198	1,131,266
Fixed maturity securities - trading	40,282	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	503,789	504,365
Fixed maturity securities - trading	34,958	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	626,409	626,901
Fixed maturity securities - trading	5,324	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