FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of Fair Value Measurements for Level 3 (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Transfers
Securities transferred into Level 3		$ 75,700,000
Transferred out of Level 3	$ 28,900,000	221,500,000
Transfers from Level 2 to Level 1	0	12,200,000
Transfers from Level 1 to Level 2	0	100,000
Transfers from Level 1 to Level 2, Liabilities		120,000,000
Other long-term investments
Transfers
Transfers from Level 1 to Level 2		169,400,000
Level 3
Assets:
Beginning Balance	1,510,043,000
Issuances	0	0
Settlements	0	0
Ending Balance	1,374,044,000	1,510,043,000
Liabilities:
Beginning Balance	659,459,000	678,068,000
Total Realized and Unrealized Gains Included in Earnings	93,071,000	499,894,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(286,119,000)	(489,325,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	623,000	555,000
Settlements	8,768,000	9,844,000
Transfers in/out of Level 3	0	0
Other	0	1,249,000
Ending Balance	844,362,000	659,459,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(189,047,000)	13,713,000
Level 3 | Annuity account
Liabilities:
Beginning Balance	87,616,000	92,512,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(4,001,000)	(3,144,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	623,000	555,000
Settlements	8,768,000	9,844,000
Transfers in/out of Level 3	0	0
Other	0	1,249,000
Ending Balance	83,472,000	87,616,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other liabilities
Liabilities:
Beginning Balance	571,843,000	585,556,000
Total Realized and Unrealized Gains Included in Earnings	93,071,000	499,894,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(282,118,000)	(486,181,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	760,890,000	571,843,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(189,047,000)	13,713,000
Level 3 | Total investments
Assets:
Beginning Balance	1,510,043,000	1,826,883,000
Total Realized and Unrealized Gains Included in Earnings	32,438,000	90,991,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	42,908,000	83,446,000
Total Realized and Unrealized Losses Included in Earnings	(19,324,000)	(59,720,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(24,073,000)	(64,574,000)
Purchases	143,784,000	169,403,000
Sales	(281,289,000)	(379,212,000)
Transfers in/out of Level 3	(28,937,000)	(145,765,000)
Other	(1,506,000)	(11,409,000)
Ending Balance	1,374,044,000	1,510,043,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	15,209,000	28,190,000
Level 3 | Residential mortgage-backed securities | Available-for-sale securities
Assets:
Beginning Balance	3,000	3,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	83,000	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	11,862,000	0
Sales	(3,000)	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(11,944,000)	0
Other	(1,000)	0
Ending Balance	0	3,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Residential mortgage-backed securities | Trading securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Commercial mortgage-backed securities | Available-for-sale securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	7,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	(1,750,000)
Purchases	0	25,607,000
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(23,844,000)
Other	0	(20,000)
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Commercial mortgage-backed securities | Trading securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other asset-backed securities | Available-for-sale securities
Assets:
Beginning Balance	562,604,000	587,031,000
Total Realized and Unrealized Gains Included in Earnings	1,409,000	6,859,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	15,136,000	42,865,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(10,931,000)	(29,673,000)
Purchases	100,000	30,441,000
Sales	(59,175,000)	(79,314,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(6,643,000)	7,457,000
Other	1,865,000	(3,062,000)
Ending Balance	504,365,000	562,604,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other asset-backed securities | Trading securities
Assets:
Beginning Balance	84,563,000	152,912,000
Total Realized and Unrealized Gains Included in Earnings	3,768,000	5,386,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(1,157,000)	(4,790,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(52,835,000)	(70,270,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	172,000
Other	883,000	1,153,000
Ending Balance	35,222,000	84,563,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	3,483,000	594,000
Level 3 | U.S. government-related securities | Available-for-sale securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | U.S. government-related securities | Trading securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | States, municipals, and political subdivisions | Available-for-sale securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | States, municipals, and political subdivisions | Trading securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other government-related securities | Available-for-sale securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other government-related securities | Trading securities
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Corporate securities | Available-for-sale securities
Assets:
Beginning Balance	664,046,000	902,119,000
Total Realized and Unrealized Gains Included in Earnings	0	925,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	27,637,000	40,574,000
Total Realized and Unrealized Losses Included in Earnings	0	(4,135,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,089,000)	(33,151,000)
Purchases	131,822,000	102,426,000
Sales	(169,002,000)	(225,557,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(10,353,000)	(109,792,000)
Other	(4,160,000)	(9,363,000)
Ending Balance	626,901,000	664,046,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Corporate securities | Trading securities
Assets:
Beginning Balance	5,492,000	18,225,000
Total Realized and Unrealized Gains Included in Earnings	101,000	713,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(58,000)	(442,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	10,906,000
Sales	0	(4,071,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(19,722,000)
Other	(93,000)	(117,000)
Ending Balance	5,442,000	5,492,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	44,000	101,000
Level 3 | Fixed maturities
Assets:
Beginning Balance	1,316,708,000	1,660,290,000
Total Realized and Unrealized Gains Included in Earnings	5,278,000	13,883,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	42,856,000	83,446,000
Total Realized and Unrealized Losses Included in Earnings	(1,215,000)	(9,367,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(24,020,000)	(64,574,000)
Purchases	143,784,000	169,380,000
Sales	(281,015,000)	(379,212,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(28,940,000)	(145,729,000)
Other	(1,506,000)	(11,409,000)
Ending Balance	1,171,930,000	1,316,708,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	3,527,000	695,000
Level 3 | Fixed maturities | Available-for-sale securities
Assets:
Beginning Balance	1,226,653,000	1,489,153,000
Total Realized and Unrealized Gains Included in Earnings	1,409,000	7,784,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	42,856,000	83,446,000
Total Realized and Unrealized Losses Included in Earnings	0	(4,135,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(24,020,000)	(64,574,000)
Purchases	143,784,000	158,474,000
Sales	(228,180,000)	(304,871,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(28,940,000)	(126,179,000)
Other	(2,296,000)	(12,445,000)
Ending Balance	1,131,266,000	1,226,653,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Fixed maturities | Trading securities
Assets:
Beginning Balance	90,055,000	171,137,000
Total Realized and Unrealized Gains Included in Earnings	3,869,000	6,099,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(1,215,000)	(5,232,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	10,906,000
Sales	(52,835,000)	(74,341,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(19,550,000)
Other	790,000	1,036,000
Ending Balance	40,664,000	90,055,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	3,527,000	695,000
Level 3 | Equity securities
Assets:
Beginning Balance	69,010,000	69,763,000
Total Realized and Unrealized Gains Included in Earnings	2,000	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	52,000	0
Total Realized and Unrealized Losses Included in Earnings	(2,630,000)	(740,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(53,000)	0
Purchases	0	23,000
Sales	(274,000)	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	3,000	(36,000)
Other	0	0
Ending Balance	66,110,000	69,010,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	3,000	0
Level 3 | Other long-term investments
Assets:
Beginning Balance	124,325,000	96,830,000
Total Realized and Unrealized Gains Included in Earnings	27,158,000	77,108,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(15,479,000)	(49,613,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	136,004,000	124,325,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	11,679,000	27,495,000
Level 3 | Short-term investments
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ 0	$ 0