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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Transfers
Transfers into Level 3	$ 0	$ 27,200,000	$ 0	$ 71,300,000
Securities transferred from level 3 to level 2	2,200,000	44,000,000	11,500,000	150,200,000
Transfers from Level 2 to Level 1	0	0	0	12,200,000
Transfers from Level 1	0	0	0	100,000
Level 3
Assets:
Beginning Balance	1,497,709,000	1,664,355,000	1,510,043,000	1,826,883,000
Total Realized and Unrealized Gains Included in Earnings	305,000	4,339,000	14,874,000	11,734,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,046,000	16,958,000	21,349,000	31,880,000
Total Realized and Unrealized Losses Included in Earnings	(13,680,000)	(19,273,000)	(18,854,000)	(54,735,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,517,000)	(9,044,000)	(5,630,000)	(28,388,000)
Purchases	55,354,000	30,085,000	92,613,000	46,085,000
Sales	(68,202,000)	(65,360,000)	(128,412,000)	(146,597,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(2,220,000)	(16,773,000)	(11,507,000)	(78,838,000)
Other	(550,000)	(1,700,000)	(231,000)	(4,437,000)
Ending Balance	1,474,245,000	1,603,587,000	1,474,245,000	1,603,587,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(13,169,000)	(15,858,000)	(629,000)	(46,485,000)
Liabilities:
Beginning Balance	673,489,000	891,904,000	659,459,000	678,068,000
Total Realized and Unrealized Gains Included in Earnings	0	15,217,000	44,263,000	15,585,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(114,034,000)	(186,050,000)	(172,641,000)	(403,209,000)
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	212,000	63,000	392,000	250,000
Settlements	1,643,000	1,896,000	3,911,000	5,038,000
Transfers in/out of Level 3	0	0	0	0
Other	0	0	0	0
Ending Balance	788,312,000	1,060,904,000	788,312,000	1,060,904,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(115,144,000)	(170,303,000)	(130,375,000)	(386,528,000)
Level 3 | Annuity account
Liabilities:
Beginning Balance	86,415,000	90,123,000	87,616,000	92,512,000
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	1,110,000	(530,000)	1,997,000	(1,096,000)
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	212,000	63,000	392,000	250,000
Settlements	1,643,000	1,896,000	3,911,000	5,038,000
Transfers in/out of Level 3	0	0	0	0
Other	0	0	0	0
Ending Balance	86,094,000	88,820,000	86,094,000	88,820,000
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	587,074,000	801,781,000	571,843,000	585,556,000
Total Realized and Unrealized Gains Included in Earnings	0	15,217,000	44,263,000	15,585,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(115,144,000)	(185,520,000)	(174,638,000)	(402,113,000)
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	702,218,000	972,084,000	702,218,000	972,084,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(115,144,000)	(170,303,000)	(130,375,000)	(386,528,000)
Level 3 | Total investments
Assets:
Beginning Balance	1,497,709,000	1,664,355,000	1,510,043,000	1,826,883,000
Total Realized and Unrealized Gains Included in Earnings	305,000	4,339,000	14,874,000	11,734,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,046,000	16,958,000	21,349,000	31,880,000
Total Realized and Unrealized Losses Included in Earnings	(13,680,000)	(19,273,000)	(18,854,000)	(54,735,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,517,000)	(9,044,000)	(5,630,000)	(28,388,000)
Purchases	55,354,000	30,085,000	92,613,000	46,085,000
Sales	(68,202,000)	(65,360,000)	(128,412,000)	(146,597,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(2,220,000)	(16,773,000)	(11,507,000)	(78,838,000)
Other	(550,000)	(1,700,000)	(231,000)	(4,437,000)
Ending Balance	1,474,245,000	1,603,587,000	1,474,245,000	1,603,587,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(13,169,000)	(15,858,000)	(629,000)	(46,485,000)
Level 3 | Fixed maturities
Assets:
Beginning Balance	1,297,897,000	1,527,931,000	1,316,708,000	1,660,290,000
Total Realized and Unrealized Gains Included in Earnings	245,000	2,771,000	3,753,000	10,166,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,027,000	16,958,000	21,328,000	31,880,000
Total Realized and Unrealized Losses Included in Earnings	(215,000)	(8,000)	(801,000)	(5,336,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,517,000)	(9,044,000)	(5,630,000)	(28,388,000)
Purchases	55,354,000	30,063,000	92,613,000	46,063,000
Sales	(68,098,000)	(65,360,000)	(128,308,000)	(146,597,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(2,220,000)	(16,773,000)	(11,510,000)	(78,802,000)
Other	(551,000)	(1,700,000)	(231,000)	(4,438,000)
Ending Balance	1,287,922,000	1,484,838,000	1,287,922,000	1,484,838,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	236,000	1,839,000	3,672,000	1,346,000
Level 3 | Equity securities
Assets:
Beginning Balance	66,384,000	69,728,000	69,010,000	69,763,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	19,000	0	21,000	0
Total Realized and Unrealized Losses Included in Earnings	0	0	(2,630,000)	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	22,000	0	22,000
Sales	(104,000)	0	(104,000)	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	3,000	(36,000)
Other	1,000	0	0	1,000
Ending Balance	66,300,000	69,750,000	66,300,000	69,750,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	1,000	0
Level 3 | Other long-term investments
Assets:
Beginning Balance	133,428,000	66,696,000	124,325,000	96,830,000
Total Realized and Unrealized Gains Included in Earnings	60,000	1,568,000	11,121,000	1,568,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(13,465,000)	(19,265,000)	(15,423,000)	(49,399,000)
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	120,023,000	48,999,000	120,023,000	48,999,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(13,405,000)	(17,697,000)	(4,302,000)	(47,831,000)
Level 3 | Available-for-sale securities | Fixed maturities
Assets:
Beginning Balance	1,223,641,000	1,371,571,000	1,226,653,000	1,489,153,000
Total Realized and Unrealized Gains Included in Earnings	0	925,000	0	7,784,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,027,000	16,958,000	21,328,000	31,880,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	(4,135,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,517,000)	(9,044,000)	(5,630,000)	(28,388,000)
Purchases	55,354,000	19,155,000	92,613,000	35,155,000
Sales	(54,078,000)	(64,752,000)	(94,980,000)	(140,314,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(2,220,000)	(16,773,000)	(11,510,000)	(70,495,000)
Other	(728,000)	(1,753,000)	(995,000)	(4,353,000)
Ending Balance	1,227,479,000	1,316,287,000	1,227,479,000	1,316,287,000
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	3,000	3,000	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	0	0	0	0
Purchases	11,862,000	0	11,862,000	0
Sales	0	0	(3,000)	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	11,862,000	3,000	11,862,000	3,000
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	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	0	0	0	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	556,936,000	537,831,000	562,604,000	587,031,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	6,859,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	769,000	3,530,000	769,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,257,000)	(8,350,000)	(5,088,000)	(21,407,000)
Purchases	0	9,597,000	0	9,597,000
Sales	(12,000)	(7,629,000)	(2,027,000)	(58,449,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	(6,643,000)	7,457,000
Other	296,000	923,000	587,000	1,284,000
Ending Balance	552,963,000	533,141,000	552,963,000	533,141,000
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	666,705,000	833,737,000	664,046,000	902,119,000
Total Realized and Unrealized Gains Included in Earnings	0	925,000	0	925,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,027,000	16,189,000	17,798,000	31,111,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	(4,135,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(260,000)	(694,000)	(542,000)	(6,981,000)
Purchases	43,492,000	9,558,000	80,751,000	25,558,000
Sales	(54,066,000)	(57,123,000)	(92,950,000)	(81,865,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(2,220,000)	(16,773,000)	(4,867,000)	(77,952,000)
Other	(1,024,000)	(2,676,000)	(1,582,000)	(5,637,000)
Ending Balance	662,654,000	783,143,000	662,654,000	783,143,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Trading securities
Assets:
Total Realized and Unrealized Gains Included in Other Comprehensive Income				0
Total Realized and Unrealized Losses Included in Other Comprehensive Income				0
Issuances				0
Settlements				0
Level 3 | Trading securities | Fixed maturities
Assets:
Beginning Balance	74,256,000	156,360,000	90,055,000	171,137,000
Total Realized and Unrealized Gains Included in Earnings	245,000	1,846,000	3,753,000	2,382,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(215,000)	(8,000)	(801,000)	(1,201,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	10,908,000	0	10,908,000
Sales	(14,020,000)	(608,000)	(33,328,000)	(6,283,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	(8,307,000)
Other	177,000	53,000	764,000	(85,000)
Ending Balance	60,443,000	168,551,000	60,443,000	168,551,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	236,000	1,839,000	3,672,000	1,346,000
Level 3 | Trading securities | Other asset-backed securities
Assets:
Beginning Balance	68,752,000	150,683,000	84,563,000	152,912,000
Total Realized and Unrealized Gains Included in Earnings	205,000	1,822,000	3,679,000	2,050,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(215,000)	(8,000)	(801,000)	(942,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	(14,020,000)	(609,000)	(33,328,000)	(2,212,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	172,000
Other	201,000	76,000	810,000	(16,000)
Ending Balance	54,923,000	151,964,000	54,923,000	151,964,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	196,000	1,814,000	3,598,000	1,105,000
Level 3 | Trading securities | Corporate securities
Assets:
Beginning Balance	5,504,000	5,677,000	5,492,000	18,225,000
Total Realized and Unrealized Gains Included in Earnings	40,000	24,000	74,000	332,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0
Total Realized and Unrealized Losses Included in Earnings	0	0	0	(259,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0
Purchases	0	10,908,000	0	10,908,000
Sales	0	1,000	0	(4,071,000)
Issuances	0	0	0
Settlements	0	0	0
Transfers in/out of Level 3	0	0	0	(8,479,000)
Other	(24,000)	(23,000)	(46,000)	(69,000)
Ending Balance	5,520,000	16,587,000	5,520,000	16,587,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ 40,000	$ 25,000	$ 74,000	$ 241,000