FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Available-for-sale securities, fair value	$ 37,543,164	$ 36,328,836
Total fixed maturity securities	39,175,025	37,996,577
Equity securities	750,141	716,017
Other long-term investments	429,369	417,769
Short-term investments	362,048	315,834
Assets related to separate accounts
Variable annuity	13,616,259	13,244,252
Variable universal life	959,638	895,925
Liabilities:
Other liabilities	203,002	176,979
Fixed maturities
Assets:
Available-for-sale securities, fair value	36,481,926	35,356,717
Trading securities	2,700,000	2,600,000
Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,992,315	1,889,607
Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,802,778	1,782,497
Other asset-backed securities
Assets:
Available-for-sale securities, fair value	1,182,003	1,211,533
States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,689,321	1,656,503
Other government-related securities
Assets:
Available-for-sale securities, fair value	249,458	237,926
Corporate securities
Assets:
Available-for-sale securities, fair value	28,176,875	27,224,649
Level 3 | Other asset-backed securities
Assets related to separate accounts
Total assets measured at fair value on a recurring basis	552,816	553,308
Level 3 | Corporate securities
Assets related to separate accounts
Total assets measured at fair value on a recurring basis	634,340	638,279
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	39,175,025	37,996,577
Equity securities	750,141	716,017
Other long-term investments	522,763	533,433
Short-term investments	362,048	315,834
Total investments	40,809,977	39,561,861
Cash	244,197	214,439
Assets related to separate accounts
Variable annuity	13,616,259	13,244,252
Variable universal life	959,638	895,925
Total assets measured at fair value on a recurring basis	55,630,071	53,916,477
Liabilities:
Annuity account balances	86,094	87,616
Other liabilities	805,542	674,048
Total liabilities measured at fair value on a recurring basis	891,636	761,664
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Available-for-sale securities, fair value	36,481,926	35,356,717
Trading securities	2,693,099	2,639,860
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,992,315	1,889,607
Trading securities	264,109	255,027
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,802,778	1,782,497
Trading securities	152,522	149,683
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	1,182,003	1,211,533
Trading securities	165,775	200,084
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	1,296,108	1,268,159
Trading securities	26,101	26,961
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,689,321	1,656,503
Trading securities	323,720	316,519
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Available-for-sale securities, fair value	249,458	237,926
Trading securities	63,763	63,012
Measured at fair value on a recurring basis | Corporate securities
Assets:
Available-for-sale securities, fair value	28,176,875	27,224,649
Trading securities	1,693,996	1,624,589
Measured at fair value on a recurring basis | Preferred Stock
Assets:
Available-for-sale securities, fair value	93,068	85,843
Trading securities	3,113	3,985
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,109,418	1,095,210
Equity securities	684,455	650,231
Other long-term investments	68,323	82,420
Short-term investments	333,255	313,835
Total investments	2,195,451	2,141,696
Cash	244,197	214,439
Assets related to separate accounts
Variable annuity	13,616,259	13,244,252
Variable universal life	959,638	895,925
Total assets measured at fair value on a recurring basis	17,015,545	16,496,312
Liabilities:
Annuity account balances	0	0
Other liabilities	6,586	13,004
Total liabilities measured at fair value on a recurring basis	6,586	13,004
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Available-for-sale securities, fair value	1,084,696	1,068,801
Trading securities	24,722	26,409
Measured at fair value on a recurring basis | Level 1 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	1,010,562	1,002,020
Trading securities	21,609	22,424
Measured at fair value on a recurring basis | Level 1 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Other government-related securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Corporate securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Preferred Stock
Assets:
Available-for-sale securities, fair value	74,134	66,781
Trading securities	3,113	3,985
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	36,777,685	35,584,659
Equity securities	0	0
Other long-term investments	361,195	335,497
Short-term investments	28,793	1,999
Total investments	37,167,673	35,922,155
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	37,167,673	35,922,155
Liabilities:
Annuity account balances	0	0
Other liabilities	266,193	255,241
Total liabilities measured at fair value on a recurring basis	266,193	255,241
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Available-for-sale securities, fair value	34,169,751	33,061,263
Trading securities	2,607,934	2,523,396
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,980,453	1,889,604
Trading securities	264,109	255,027
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	1,802,778	1,782,497
Trading securities	152,522	149,683
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	629,040	648,929
Trading securities	110,852	115,521
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	285,546	266,139
Trading securities	4,492	4,537
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	1,689,321	1,656,503
Trading securities	323,720	316,519
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Available-for-sale securities, fair value	249,458	237,926
Trading securities	63,763	63,012
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Available-for-sale securities, fair value	27,514,221	26,560,603
Trading securities	1,688,476	1,619,097
Measured at fair value on a recurring basis | Level 2 | Preferred Stock
Assets:
Available-for-sale securities, fair value	18,934	19,062
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,287,922	1,316,708
Equity securities	65,686	65,786
Other long-term investments	93,245	115,516
Short-term investments	0	0
Total investments	1,446,853	1,498,010
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	1,446,853	1,498,010
Liabilities:
Annuity account balances	86,094	87,616
Other liabilities	532,763	405,803
Total liabilities measured at fair value on a recurring basis	618,857	493,419
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Available-for-sale securities, fair value	1,227,479	1,226,653
Trading securities	60,443	90,055
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Available-for-sale securities, fair value	11,862	3
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Other asset-backed securities
Assets:
Available-for-sale securities, fair value	552,963	562,604
Trading securities	54,923	84,563
Measured at fair value on a recurring basis | Level 3 | U.S. government-related securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Other government-related securities
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Corporate securities
Assets:
Available-for-sale securities, fair value	662,654	664,046
Trading securities	5,520	5,492
Measured at fair value on a recurring basis | Level 3 | Preferred Stock
Assets:
Available-for-sale securities, fair value	0	0
Trading securities	$ 0	$ 0