FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Available-for-sale securities	$ 36,966,122	$ 36,570,665
Total fixed maturity securities	38,575,433	38,183,337
Equity securities	792,231	754,489
Other long-term investments	545,645	542,243
Short-term investments	299,167	332,431
Assets related to separate accounts
Variable annuity	13,512,921	13,244,252
Variable universal life	935,427	895,925
Fixed maturities
Assets:
Available-for-sale securities	35,931,511	35,543,477
Trading securities	2,600,000	2,600,000
Residential mortgage-backed securities
Assets:
Available-for-sale securities	1,970,192	1,898,483
Commercial mortgage-backed securities
Assets:
Available-for-sale securities	1,814,663	1,811,470
Other asset-backed securities
Assets:
Available-for-sale securities	1,182,450	1,211,533
States, municipals, and political subdivisions
Assets:
Available-for-sale securities	1,672,724	1,656,503
Other government-related securities
Assets:
Available-for-sale securities	245,201	239,921
Corporate securities
Assets:
Available-for-sale securities	27,682,212	27,371,565
Level 3 | Other asset-backed securities
Assets:
Total investments	556,782	553,308
Level 3 | Corporate securities
Assets:
Total investments	639,904	638,279
Measured at fair value on a recurring basis
Assets:
Total fixed maturity securities	38,575,433	38,183,337
Equity securities	792,231	754,489
Other long-term investments	545,645	542,243
Short-term investments	299,167	332,431
Total investments	40,212,476	39,812,500
Cash	409,377	348,182
Other assets	27,784	23,830
Assets related to separate accounts
Variable annuity	13,512,921	13,244,252
Variable universal life	935,427	895,925
Total assets measured at fair value on a recurring basis	55,097,985	54,324,689
Liabilities:
Annuity account balances	86,415	87,616
Other liabilities	802,493	748,821
Total liabilities measured at fair value on a recurring basis	888,908	836,437
Measured at fair value on a recurring basis | Other liabilities
Liabilities:
Transfer from Level 1 to Level 2		120,000
Measured at fair value on a recurring basis | Other long-term investments
Liabilities:
Transfer from Level 1 to Level 2		169,400
Measured at fair value on a recurring basis | Fixed maturities
Assets:
Available-for-sale securities	35,931,511	35,543,477
Trading securities	2,643,922	2,639,860
Measured at fair value on a recurring basis | Residential mortgage-backed securities
Assets:
Available-for-sale securities	1,970,192	1,898,483
Trading securities	255,779	255,027
Measured at fair value on a recurring basis | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	1,814,663	1,811,470
Trading securities	154,760	149,683
Measured at fair value on a recurring basis | Other asset-backed securities
Assets:
Available-for-sale securities	1,182,450	1,211,533
Trading securities	181,300	200,084
Measured at fair value on a recurring basis | U.S. government-related securities
Assets:
Available-for-sale securities	1,274,795	1,268,159
Trading securities	48,975	26,961
Measured at fair value on a recurring basis | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	1,672,724	1,656,503
Trading securities	312,095	316,519
Measured at fair value on a recurring basis | Other government-related securities
Assets:
Available-for-sale securities	245,201	239,921
Trading securities	63,369	63,012
Measured at fair value on a recurring basis | Corporate securities
Assets:
Available-for-sale securities	27,682,212	27,371,565
Trading securities	1,623,864	1,624,589
Measured at fair value on a recurring basis | Preferred stock
Assets:
Available-for-sale securities	89,274	85,843
Trading securities	3,780	3,985
Measured at fair value on a recurring basis | Level 1
Assets:
Total fixed maturity securities	1,128,264	1,095,210
Equity securities	725,811	685,443
Other long-term investments	57,787	82,420
Short-term investments	255,251	328,829
Total investments	2,167,113	2,191,902
Cash	409,377	348,182
Other assets	27,784	23,830
Assets related to separate accounts
Variable annuity	13,512,921	13,244,252
Variable universal life	935,427	895,925
Total assets measured at fair value on a recurring basis	17,052,622	16,704,091
Liabilities:
Annuity account balances	0	0
Other liabilities	12,152	13,004
Total liabilities measured at fair value on a recurring basis	12,152	13,004
Measured at fair value on a recurring basis | Level 1 | Fixed maturities
Assets:
Available-for-sale securities	1,080,026	1,068,801
Trading securities	48,238	26,409
Measured at fair value on a recurring basis | Level 1 | Residential mortgage-backed securities
Assets:
Available-for-sale securities	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	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	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	1,009,732	1,002,020
Trading securities	44,458	22,424
Measured at fair value on a recurring basis | Level 1 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	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	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Corporate securities
Assets:
Available-for-sale securities	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 1 | Preferred stock
Assets:
Available-for-sale securities	70,294	66,781
Trading securities	3,780	3,985
Measured at fair value on a recurring basis | Level 2
Assets:
Total fixed maturity securities	36,149,272	35,771,419
Equity securities	36	36
Other long-term investments	354,430	335,498
Short-term investments	43,916	3,602
Total investments	36,547,654	36,110,555
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	36,547,654	36,110,555
Liabilities:
Annuity account balances	0	0
Other liabilities	203,267	163,974
Total liabilities measured at fair value on a recurring basis	203,267	163,974
Measured at fair value on a recurring basis | Level 2 | Fixed maturities
Assets:
Available-for-sale securities	33,627,844	33,248,023
Trading securities	2,521,428	2,523,396
Measured at fair value on a recurring basis | Level 2 | Residential mortgage-backed securities
Assets:
Available-for-sale securities	1,970,192	1,898,480
Trading securities	255,779	255,027
Measured at fair value on a recurring basis | Level 2 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	1,814,663	1,811,470
Trading securities	154,760	149,683
Measured at fair value on a recurring basis | Level 2 | Other asset-backed securities
Assets:
Available-for-sale securities	625,514	648,929
Trading securities	112,548	115,521
Measured at fair value on a recurring basis | Level 2 | U.S. government-related securities
Assets:
Available-for-sale securities	265,063	266,139
Trading securities	4,517	4,537
Measured at fair value on a recurring basis | Level 2 | States, municipals, and political subdivisions
Assets:
Available-for-sale securities	1,672,724	1,656,503
Trading securities	312,095	316,519
Measured at fair value on a recurring basis | Level 2 | Other government-related securities
Assets:
Available-for-sale securities	245,201	239,921
Trading securities	63,369	63,012
Measured at fair value on a recurring basis | Level 2 | Corporate securities
Assets:
Available-for-sale securities	27,015,507	26,707,519
Trading securities	1,618,360	1,619,097
Measured at fair value on a recurring basis | Level 2 | Preferred stock
Assets:
Available-for-sale securities	18,980	19,062
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3
Assets:
Total fixed maturity securities	1,297,897	1,316,708
Equity securities	66,384	69,010
Other long-term investments	133,428	124,325
Short-term investments	0	0
Total investments	1,497,709	1,510,043
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	1,497,709	1,510,043
Liabilities:
Annuity account balances	86,415	87,616
Other liabilities	587,074	571,843
Total liabilities measured at fair value on a recurring basis	673,489	659,459
Measured at fair value on a recurring basis | Level 3 | Fixed maturities
Assets:
Available-for-sale securities	1,223,641	1,226,653
Trading securities	74,256	90,055
Measured at fair value on a recurring basis | Level 3 | Residential mortgage-backed securities
Assets:
Available-for-sale securities	0	3
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Commercial mortgage-backed securities
Assets:
Available-for-sale securities	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	556,936	562,604
Trading securities	68,752	84,563
Measured at fair value on a recurring basis | Level 3 | U.S. government-related securities
Assets:
Available-for-sale securities	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	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	0	0
Trading securities	0	0
Measured at fair value on a recurring basis | Level 3 | Corporate securities
Assets:
Available-for-sale securities	666,705	664,046
Trading securities	5,504	5,492
Measured at fair value on a recurring basis | Level 3 | Preferred stock
Assets:
Available-for-sale securities	0	0
Trading securities	$ 0	$ 0