FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of Beginning and Ending Balances for Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of the beginning and ending balances for fair value measurements for which the Company has used significant unobservable inputs (Level 3)
Transfers into Level 3	$ 11,100,000	$ 0	$ 11,100,000	$ 0
Level 3
Assets:
Beginning Balance	1,380,893,000	1,497,709,000	1,374,044,000	1,510,043,000
Total Realized and Unrealized Gains Included in Earnings	12,822,000	305,000	21,880,000	14,874,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	17,015,000	10,046,000	18,928,000	21,349,000
Total Realized and Unrealized Losses Included in Earnings	(4,129,000)	(13,680,000)	(4,816,000)	(18,854,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(8,582,000)	(4,517,000)	(22,317,000)	(5,630,000)
Purchases	118,973,000	55,354,000	153,973,000	92,613,000
Sales	(26,194,000)	(68,202,000)	(50,239,000)	(128,412,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(7,508,000)	(2,220,000)	(7,508,000)	(11,507,000)
Other	(908,000)	(550,000)	(1,563,000)	(231,000)
Ending Balance	1,482,382,000	1,474,245,000	1,482,382,000	1,474,245,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	8,693,000	(13,169,000)	17,063,000	(629,000)
Liabilities:
Beginning Balance	702,501,000	673,489,000	844,362,000	659,459,000
Total Realized and Unrealized Gains Included in Earnings	96,138,000	0	257,456,000	44,263,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(49,011,000)	(114,034,000)	(71,335,000)	(176,635,000)
Total Realized and Unrealized Losses Included In Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	89,000	212,000	530,000	392,000
Settlements	2,449,000	1,643,000	5,757,000	3,911,000
Transfers in/out of Level 3	0	0	0	0
Other	0	0	0	0
Ending Balance	653,014,000	788,312,000	653,014,000	788,312,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	48,186,000	(115,144,000)	187,974,000	(130,375,000)
Level 3 | Annuity account
Liabilities:
Beginning Balance	81,399,000	86,415,000	83,472,000	87,616,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(1,059,000)	1,110,000	(1,853,000)	(1,997,000)
Total Realized and Unrealized Losses Included In Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	89,000	212,000	530,000	392,000
Settlements	2,449,000	1,643,000	5,757,000	3,911,000
Transfers in/out of Level 3	0	0	0	0
Other	0	0	0	0
Ending Balance	80,098,000	86,094,000	80,098,000	86,094,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Other liabilities
Liabilities:
Beginning Balance	621,102,000	587,074,000	760,890,000	571,843,000
Total Realized and Unrealized Gains Included in Earnings	96,138,000	0	257,456,000	44,263,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(47,952,000)	(115,144,000)	(69,482,000)	(174,638,000)
Total Realized and Unrealized Losses Included In Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	0
Other	0	0	0	0
Ending Balance	572,916,000	702,218,000	572,916,000	702,218,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	48,186,000	(115,144,000)	187,974,000	(130,375,000)
Level 3 | Total investments
Assets:
Beginning Balance	1,380,893,000	1,497,709,000	1,374,044,000	1,510,043,000
Total Realized and Unrealized Gains Included in Earnings	12,822,000	305,000	21,880,000	14,874,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	17,015,000	10,046,000	18,928,000	21,349,000
Total Realized and Unrealized Losses Included in Earnings	(4,129,000)	(13,680,000)	(4,816,000)	(18,854,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(8,582,000)	(4,517,000)	(22,317,000)	(5,630,000)
Purchases	118,973,000	55,354,000	153,973,000	92,613,000
Sales	(26,194,000)	(68,202,000)	(50,239,000)	(128,412,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(7,508,000)	(2,220,000)	(7,508,000)	(11,507,000)
Other	(908,000)	(550,000)	(1,563,000)	(231,000)
Ending Balance	1,482,382,000	1,474,245,000	1,482,382,000	1,474,245,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	8,693,000	(13,169,000)	17,063,000	(629,000)
Level 3 | Fixed maturities
Assets:
Beginning Balance	1,170,480,000	1,297,897,000	1,171,930,000	1,316,708,000
Total Realized and Unrealized Gains Included in Earnings	0	245,000	194,000	3,753,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	17,015,000	10,027,000	18,928,000	21,328,000
Total Realized and Unrealized Losses Included in Earnings	(3,615,000)	(215,000)	(3,737,000)	(801,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(8,582,000)	(4,517,000)	(22,317,000)	(5,630,000)
Purchases	118,937,000	55,354,000	153,937,000	92,613,000
Sales	(26,194,000)	(68,098,000)	(50,239,000)	(128,308,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(7,508,000)	(2,220,000)	(7,508,000)	(11,510,000)
Other	(908,000)	(551,000)	(1,563,000)	(231,000)
Ending Balance	1,259,625,000	1,287,922,000	1,259,625,000	1,287,922,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(3,615,000)	236,000	(3,544,000)	3,672,000
Level 3 | Equity securities
Assets:
Beginning Balance	66,061,000	66,384,000	66,110,000	69,010,000
Total Realized and Unrealized Gains Included in Earnings	1,000	0	1,000	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	19,000	0	21,000
Total Realized and Unrealized Losses Included in Earnings	(15,000)	0	(64,000)	(2,630,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	36,000	0	36,000	0
Sales	0	(104,000)	0	(104,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	3,000
Other	0	1,000	0	0
Ending Balance	66,083,000	66,300,000	66,083,000	66,300,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(14,000)	0	(63,000)	1,000
Level 3 | Other long-term investments
Assets:
Beginning Balance	144,352,000	133,428,000	136,004,000	124,325,000
Total Realized and Unrealized Gains Included in Earnings	12,821,000	60,000	21,685,000	11,121,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(499,000)	(13,465,000)	(1,015,000)	(15,423,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	0
Other	0	0	0	0
Ending Balance	156,674,000	120,023,000	156,674,000	120,023,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	12,322,000	(13,405,000)	20,670,000	(4,302,000)
Level 3 | Short-term investments
Assets:
Beginning Balance			0
Total Realized and Unrealized Gains Included in Earnings			0
Total Realized and Unrealized Gains Included in Other Comprehensive Income			0
Total Realized and Unrealized Losses Included in Earnings			0
Total Realized and Unrealized Losses Included in Other Comprehensive Income			0
Purchases			0
Sales			0
Issuances			0
Settlements			0
Transfers in/out of Level 3			0
Other			0
Ending Balance	0		0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date			0
Level 3 | Available-for-sale securities | Fixed maturities
Assets:
Beginning Balance	1,130,198,000	1,223,641,000	1,131,266,000	1,226,653,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	17,015,000	10,027,000	18,928,000	21,328,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(8,582,000)	(4,517,000)	(22,317,000)	(5,630,000)
Purchases	114,337,000	55,354,000	149,337,000	92,613,000
Sales	(14,921,000)	(54,078,000)	(38,570,000)	(94,980,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(7,672,000)	(2,220,000)	(7,672,000)	(11,510,000)
Other	(856,000)	(728,000)	(1,453,000)	(995,000)
Ending Balance	1,229,519,000	1,227,479,000	1,229,519,000	1,227,479,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Available-for-sale securities | Residential mortgage-backed securities
Assets:
Beginning Balance	0	0	0	3,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(457,000)	0	(457,000)	0
Purchases	22,225,000	11,862,000	22,225,000	11,862,000
Sales	0	0	0	(3,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	0
Other	12,000	0	12,000	0
Ending Balance	21,780,000	11,862,000	21,780,000	11,862,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Available-for-sale securities | Commercial mortgage-backed securities
Assets:
Beginning Balance	0	0	0	0
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(1,284,000)	0	(1,284,000)	0
Purchases	48,621,000	0	48,621,000	0
Sales	(94,000)	0	(94,000)	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	0
Other	(16,000)	0	(16,000)	0
Ending Balance	47,227,000	0	47,227,000	0
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Available-for-sale securities | Other asset-backed securities
Assets:
Beginning Balance	503,789,000	556,936,000	504,365,000	562,604,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	11,353,000	0	11,867,000	3,530,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,000)	(4,257,000)	(1,647,000)	(5,088,000)
Purchases	0	0	0	0
Sales	(9,000)	(12,000)	(23,000)	(2,027,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	222,000	0	222,000	(6,643,000)
Other	359,000	296,000	917,000	587,000
Ending Balance	515,701,000	552,963,000	515,701,000	552,963,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Available-for-sale securities | Corporate securities
Assets:
Beginning Balance	626,409,000	666,705,000	626,901,000	664,046,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	5,662,000	10,027,000	7,061,000	17,798,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(6,828,000)	(260,000)	(18,929,000)	(542,000)
Purchases	43,491,000	43,492,000	78,491,000	80,751,000
Sales	(14,818,000)	(54,066,000)	(38,453,000)	(92,950,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(7,894,000)	(2,220,000)	(7,894,000)	(4,867,000)
Other	(1,211,000)	(1,024,000)	(2,366,000)	(1,582,000)
Ending Balance	644,811,000	662,654,000	644,811,000	662,654,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Trading securities | Fixed maturities
Assets:
Beginning Balance	40,282,000	74,256,000	40,664,000	90,055,000
Total Realized and Unrealized Gains Included in Earnings	0	245,000	194,000	3,753,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(3,615,000)	(215,000)	(3,737,000)	(801,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	4,600,000	0	4,600,000	0
Sales	(11,273,000)	(14,020,000)	(11,669,000)	(33,328,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	164,000	0	164,000	0
Other	(52,000)	177,000	(110,000)	764,000
Ending Balance	30,106,000	60,443,000	30,106,000	60,443,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(3,615,000)	236,000	(3,544,000)	3,672,000
Level 3 | Trading securities | Other asset-backed securities
Assets:
Beginning Balance	34,958,000	68,752,000	35,222,000	84,563,000
Total Realized and Unrealized Gains Included in Earnings	0	205,000	194,000	3,679,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(3,570,000)	(215,000)	(3,598,000)	(801,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	4,600,000	0	4,600,000	0
Sales	(11,273,000)	(14,020,000)	(11,669,000)	(33,328,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	164,000	0	164,000	0
Other	(27,000)	201,000	(61,000)	810,000
Ending Balance	24,852,000	54,923,000	24,852,000	54,923,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(3,570,000)	196,000	(3,405,000)	3,598,000
Level 3 | Trading securities | Corporate securities
Assets:
Beginning Balance	5,324,000	5,504,000	5,442,000	5,492,000
Total Realized and Unrealized Gains Included in Earnings	0	40,000	0	74,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(45,000)	0	(139,000)	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	0
Other	(25,000)	(24,000)	(49,000)	(46,000)
Ending Balance	5,254,000	5,520,000	5,254,000	5,520,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ (45,000)	$ 40,000	$ (139,000)	$ 74,000