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
Securities transferred into Level 3	$ 0	$ 27,200,000	$ 0	$ 71,300,000
Securities transferred into Level 2 from Level 3	2,200,000	44,000,000	11,500,000	150,100,000
Securities transferred from Level 2 to Level 1	0	0	0	12,200,000
Securities transferred out of Level 1	0	0	0	100,000
Level 3
Assets:
Beginning Balance	1,467,580,000	1,651,483,000	1,498,010,000	1,795,178,000
Total Realized and Unrealized Gains Included in Earnings	976,000	19,046,000	9,729,000	30,658,000
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	(11,596,000)	(10,411,000)	(29,048,000)	(32,721,000)
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	47,514,000
Sales	(68,201,000)	(65,360,000)	(128,411,000)	(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)	(78,802,000)
Other	(550,000)	(1,700,000)	(231,000)	(4,438,000)
Ending Balance	1,446,853,000	1,614,284,000	1,446,853,000	1,614,284,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(10,414,000)	7,711,000	(18,599,000)	(5,547,000)
Liabilities:
Beginning Balance	525,216,000	586,361,000	493,419,000	468,360,000
Total Realized and Unrealized Gains Included in Earnings	0	15,217,000	15,280,000	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	(95,072,000)	(100,555,000)	(144,237,000)	(221,879,000)
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	618,857,000	669,866,000	618,857,000	669,866,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(93,962,000)	(84,808,000)	(126,960,000)	(205,198,000)
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	438,801,000	496,238,000	405,803,000	375,848,000
Total Realized and Unrealized Gains Included in Earnings	0	15,217,000	15,280,000	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	(93,962,000)	(100,025,000)	(142,240,000)	(220,783,000)
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	532,763,000	581,046,000	532,763,000	581,046,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(93,962,000)	(84,808,000)	(126,960,000)	(205,198,000)
Level 3 | Total investments
Assets:
Beginning Balance	1,467,580,000	1,651,483,000	1,498,010,000	1,795,178,000
Total Realized and Unrealized Gains Included in Earnings	976,000	19,046,000	9,729,000	30,658,000
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	(11,596,000)	(10,411,000)	(29,048,000)	(32,721,000)
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	47,514,000
Sales	(68,201,000)	(65,360,000)	(128,411,000)	(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)	(78,802,000)
Other	(550,000)	(1,700,000)	(231,000)	(4,438,000)
Ending Balance	1,446,853,000	1,614,284,000	1,446,853,000	1,614,284,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(10,414,000)	7,711,000	(18,599,000)	(5,547,000)
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 | Fixed maturities | Available-for-sale securities
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 | Fixed maturities | Trading Securities
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 | Residential mortgage-backed securities | Available-for-sale 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 | Commercial mortgage-backed securities | Available-for-sale 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 | Other asset-backed securities | Available-for-sale 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 | Other asset-backed securities | Trading 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 | Corporate securities | Available-for-sale 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 | Corporate securities | Trading 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	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	0
Purchases	0	10,908,000	0	10,908,000
Sales	0	1,000	0	(4,071,000)
Issuances	0	0	0	0
Settlements	0	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
Level 3 | Equity securities
Assets:
Beginning Balance	65,788,000	66,504,000	65,786,000	66,504,000
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	0	22,000	0	22,000
Sales	(103,000)	0	(103,000)	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	3,000	0
Other	1,000	0	0	0
Ending Balance	65,686,000	66,526,000	65,686,000	66,526,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0	0	0
Level 3 | Other long-term investments
Assets:
Beginning Balance	103,895,000	57,048,000	115,516,000	68,384,000
Total Realized and Unrealized Gains Included in Earnings	731,000	16,275,000	5,976,000	20,492,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(11,381,000)	(10,403,000)	(28,247,000)	(27,385,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	1,429,000
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	93,245,000	62,920,000	93,245,000	62,920,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ (10,650,000)	$ 5,872,000	$ (22,271,000)	$ (6,893,000)