FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of Fair Value Measurements for Level 3 (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Liabilities:
Transfers in/out of Level 3		$ 0
Transfers
Securities transferred into Level 3	$ 39,700,000
Transferred out of Level 3	85,700,000	28,900,000
Transfers from Level 2 to Level 1	0	0
Transfers from Level 1 to Level 2	0	0
Level 3
Assets:
Beginning Balance	1,397,914,000
Issuances	0	0
Settlements	(627,000)	0
Ending Balance	1,306,979,000	1,397,914,000
Liabilities:
Beginning Balance	681,034,000	493,419,000
Total Realized and Unrealized Gains Included in Earnings	299,366,000	35,703,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(143,436,000)	(231,463,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	623,000	623,000
Settlements	11,481,000	8,768,000
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	514,246,000	681,034,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	159,435,000	(191,759,000)
Level 3 | Variable annuity
Liabilities:
Beginning Balance	83,472,000	87,616,000
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	(3,505,000)	(4,001,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	623,000	623,000
Settlements	11,481,000	8,768,000
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	76,119,000	83,472,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other liabilities
Liabilities:
Beginning Balance	597,562,000	405,803,000
Total Realized and Unrealized Gains Included in Earnings	299,366,000	35,703,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(139,931,000)	(227,462,000)
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	438,127,000	597,562,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	159,435,000	(191,759,000)
Level 3 | Residential mortgage-backed securities | Available-for-sale securities
Assets:
Beginning Balance	0	3,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	83,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(995,000)	0
Purchases	22,225,000	11,862,000
Sales	0	(3,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(21,281,000)	(11,944,000)
Other	51,000	(1,000)
Ending Balance	0	0
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	50,000	0
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(2,497,000)	0
Purchases	48,621,000	0
Sales	(292,000)	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(45,832,000)	0
Other	(50,000)	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 | 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	504,365,000	562,604,000
Total Realized and Unrealized Gains Included in Earnings	3,716,000	1,410,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	16,503,000	15,136,000
Total Realized and Unrealized Losses Included in Earnings	(159,000)	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(25,578,000)	(10,931,000)
Purchases	0	100,000
Sales	(80,050,000)	(59,176,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	222,000	(6,643,000)
Other	2,623,000	1,865,000
Ending Balance	421,642,000	504,365,000
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	35,222,000	84,563,000
Total Realized and Unrealized Gains Included in Earnings	464,000	3,768,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(3,798,000)	(1,157,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	8,728,000	0
Sales	(14,511,000)	(52,835,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	164,000	0
Other	(213,000)	883,000
Ending Balance	26,056,000	35,222,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(3,179,000)	3,483,000
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	626,901,000	664,046,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	12,537,000	27,637,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(29,017,000)	(13,089,000)
Purchases	108,491,000	131,822,000
Sales	(97,676,000)	(169,002,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	20,721,000	(10,353,000)
Other	(3,681,000)	(4,160,000)
Ending Balance	638,276,000	626,901,000
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	5,442,000	5,492,000
Total Realized and Unrealized Gains Included in Earnings	45,000	101,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(145,000)	(58,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	999,000	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	(99,000)	(93,000)
Ending Balance	6,242,000	5,442,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(101,000)	44,000
Level 3 | Fixed Maturities
Assets:
Beginning Balance	1,171,930,000	1,316,708,000
Total Realized and Unrealized Gains Included in Earnings	4,225,000	5,279,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	29,090,000	42,856,000
Total Realized and Unrealized Losses Included in Earnings	(4,102,000)	(1,215,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(58,087,000)	(24,020,000)
Purchases	189,064,000	143,784,000
Sales	(192,529,000)	(281,016,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(46,006,000)	(28,940,000)
Other	(1,369,000)	(1,506,000)
Ending Balance	1,092,216,000	1,171,930,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(3,280,000)	3,527,000
Level 3 | Fixed Maturities | Available-for-sale securities
Assets:
Beginning Balance	1,131,266,000	1,226,653,000
Total Realized and Unrealized Gains Included in Earnings	3,716,000	1,410,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	29,090,000	42,856,000
Total Realized and Unrealized Losses Included in Earnings	(159,000)	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(58,087,000)	(24,020,000)
Purchases	179,337,000	143,784,000
Sales	(178,018,000)	(228,181,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(46,170,000)	(28,940,000)
Other	(1,057,000)	(2,296,000)
Ending Balance	1,059,918,000	1,131,266,000
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	40,664,000	90,055,000
Total Realized and Unrealized Gains Included in Earnings	509,000	3,869,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(3,943,000)	(1,215,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	9,727,000	0
Sales	(14,511,000)	(52,835,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	164,000	0
Other	(312,000)	790,000
Ending Balance	32,298,000	40,664,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(3,280,000)	3,527,000
Level 3 | Equity securities
Assets:
Beginning Balance	65,518,000	65,786,000
Total Realized and Unrealized Gains Included in Earnings	1,000	2,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(30,000)	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	36,000	0
Sales	(2,103,000)	(273,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	3,000
Other	(1,000)	0
Ending Balance	63,421,000	65,518,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	282,000	3,000
Level 3 | Other long-term investments
Assets:
Beginning Balance	160,466,000	115,516,000
Total Realized and Unrealized Gains Included in Earnings	39,118,000	73,253,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(47,615,000)	(28,303,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	(627,000)	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	151,342,000	160,466,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(9,124,000)	44,950,000
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
Level 3 | Total investments
Assets:
Beginning Balance	1,397,914,000	1,498,010,000
Total Realized and Unrealized Gains Included in Earnings	43,344,000	78,534,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	29,090,000	42,856,000
Total Realized and Unrealized Losses Included in Earnings	(51,747,000)	(29,518,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(58,087,000)	(24,020,000)
Purchases	189,100,000	143,784,000
Sales	(194,632,000)	(281,289,000)
Transfers in/out of Level 3	(46,006,000)	(28,937,000)
Other	(1,370,000)	(1,506,000)
Ending Balance	1,306,979,000	1,397,914,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ (12,122,000)	$ 48,480,000