FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of Beginning and Ending Balances for Fair Value Measurements (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Transfers
Transfers into Level 3	$ 0	$ 0	$ 0	$ 71,300,000
Securities transferred from level 3 to level 2	17,400,000	47,500,000	28,900,000	197,700,000
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	1,474,245,000	1,603,587,000	1,510,043,000	1,826,883,000
Total Realized and Unrealized Gains Included in Earnings	10,358,000	24,693,000	25,232,000	36,427,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,097,000	32,681,000	31,446,000	64,561,000
Total Realized and Unrealized Losses Included in Earnings	(355,000)	(71,000)	(19,209,000)	(54,806,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,507,000)	(3,354,000)	(10,137,000)	(31,742,000)
Purchases	5,171,000	53,934,000	97,784,000	100,019,000
Sales	(61,708,000)	(35,379,000)	(190,120,000)	(181,976,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(17,430,000)	(47,480,000)	(28,937,000)	(126,318,000)
Other	(641,000)	(5,043,000)	(872,000)	(9,480,000)
Ending Balance	1,415,230,000	1,623,568,000	1,415,230,000	1,623,568,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	10,148,000	24,283,000	9,527,000	(22,202,000)
Liabilities:
Beginning Balance	788,312,000	1,060,904,000	659,459,000	678,068,000
Total Realized and Unrealized Gains Included in Earnings	12,201,000	90,166,000	56,464,000	105,751,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(47,373,000)	(58,088,000)	(224,007,000)	(461,297,000)
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	9,000	279,000	402,000	529,000
Settlements	2,052,000	2,226,000	5,963,000	7,264,000
Transfers in/out of Level 3	0	0	0	0
Other	0	1,249,000	0	1,249,000
Ending Balance	821,441,000	1,028,128,000	821,441,000	1,028,128,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(34,195,000)	32,813,000	(164,570,000)	(353,715,000)
Level 3 | Annuity account
Liabilities:
Beginning Balance	86,094,000	88,820,000	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	(977,000)	(735,000)	(2,973,000)	(1,831,000)
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	9,000	279,000	402,000	529,000
Settlements	2,052,000	2,226,000	5,963,000	7,264,000
Transfers in/out of Level 3	0	0	0	0
Other	0	1,249,000	0	1,249,000
Ending Balance	85,028,000	88,857,000	85,028,000	88,857,000
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	702,218,000	972,084,000	571,843,000	585,556,000
Total Realized and Unrealized Gains Included in Earnings	12,201,000	90,166,000	56,464,000	105,751,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(46,396,000)	(57,353,000)	(221,034,000)	(459,466,000)
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	736,413,000	939,271,000	736,413,000	939,271,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(34,195,000)	32,813,000	(164,570,000)	(353,715,000)
Level 3 | Total investments
Assets:
Beginning Balance	1,474,245,000	1,603,587,000	1,510,043,000	1,826,883,000
Total Realized and Unrealized Gains Included in Earnings	10,358,000	24,693,000	25,232,000	36,427,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,097,000	32,681,000	31,446,000	64,561,000
Total Realized and Unrealized Losses Included in Earnings	(355,000)	(71,000)	(19,209,000)	(54,806,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,507,000)	(3,354,000)	(10,137,000)	(31,742,000)
Purchases	5,171,000	53,934,000	97,784,000	100,019,000
Sales	(61,708,000)	(35,379,000)	(190,120,000)	(181,976,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(17,430,000)	(47,480,000)	(28,937,000)	(126,318,000)
Other	(641,000)	(5,043,000)	(872,000)	(9,480,000)
Ending Balance	1,415,230,000	1,623,568,000	1,415,230,000	1,623,568,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	10,148,000	24,283,000	9,527,000	(22,202,000)
Level 3 | Fixed maturities
Assets:
Beginning Balance	1,287,922,000	1,484,838,000	1,316,708,000	1,660,290,000
Total Realized and Unrealized Gains Included in Earnings	27,000	3,641,000	3,780,000	13,807,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,066,000	32,681,000	31,394,000	64,561,000
Total Realized and Unrealized Losses Included in Earnings	(353,000)	(71,000)	(1,154,000)	(5,407,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,507,000)	(3,354,000)	(10,137,000)	(31,742,000)
Purchases	5,171,000	53,934,000	97,784,000	99,997,000
Sales	(61,539,000)	(35,379,000)	(189,847,000)	(181,976,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(17,430,000)	(47,480,000)	(28,940,000)	(126,282,000)
Other	(641,000)	(5,043,000)	(872,000)	(9,481,000)
Ending Balance	1,218,716,000	1,483,767,000	1,218,716,000	1,483,767,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(182,000)	3,231,000	3,498,000	4,577,000
Level 3 | Equity securities
Assets:
Beginning Balance	66,300,000	69,750,000	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	31,000	0	52,000	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	0	0	22,000
Sales	(169,000)	0	(273,000)	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	3,000	(36,000)
Other	0	0	0	1,000
Ending Balance	66,162,000	69,750,000	66,162,000	69,750,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	1,000	0	2,000	0
Level 3 | Other long-term investments
Assets:
Beginning Balance	120,023,000	48,999,000	124,325,000	96,830,000
Total Realized and Unrealized Gains Included in Earnings	10,331,000	21,052,000	21,452,000	22,620,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(2,000)	0	(15,425,000)	(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	130,352,000	70,051,000	130,352,000	70,051,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	10,329,000	21,052,000	6,027,000	(26,779,000)
Level 3 | Available-for-sale securities | Fixed maturities
Assets:
Beginning Balance	1,227,479,000	1,316,287,000	1,226,653,000	1,489,153,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	7,784,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	10,066,000	32,681,000	31,394,000	64,561,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	(4,135,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(4,507,000)	(3,354,000)	(10,137,000)	(31,742,000)
Purchases	5,171,000	53,934,000	97,784,000	89,089,000
Sales	(42,351,000)	(26,013,000)	(137,331,000)	(166,327,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(17,430,000)	(36,237,000)	(28,940,000)	(106,732,000)
Other	(726,000)	(5,063,000)	(1,721,000)	(9,416,000)
Ending Balance	1,177,702,000	1,332,235,000	1,177,702,000	1,332,235,000
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	11,862,000	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	83,000	0	83,000	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	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	(11,944,000)	0	(11,944,000)	0
Other	(1,000)	0	(1,000)	0
Ending Balance	0	3,000	0	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	7,000	0	7,000
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	25,607,000	0	25,607,000
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	(7,000)	0	(7,000)
Ending Balance	0	25,607,000	0	25,607,000
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 | Other asset-backed securities
Assets:
Beginning Balance	552,963,000	533,141,000	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	1,682,000	23,350,000	5,212,000	24,119,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(2,285,000)	(19,000)	(7,373,000)	(21,426,000)
Purchases	100,000	0	100,000	9,597,000
Sales	(109,000)	(12,000)	(2,136,000)	(58,461,000)
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	798,000	(3,231,000)	1,385,000	(1,947,000)
Ending Balance	553,149,000	553,229,000	553,149,000	553,229,000
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	662,654,000	783,143,000	664,046,000	902,119,000
Total Realized and Unrealized Gains Included in Earnings	0	0	0	925,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	8,301,000	9,324,000	26,099,000	40,435,000
Total Realized and Unrealized Losses Included in Earnings	0	0	0	(4,135,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(2,222,000)	(3,335,000)	(2,764,000)	(10,316,000)
Purchases	5,071,000	28,327,000	85,822,000	53,885,000
Sales	(42,242,000)	(26,001,000)	(135,192,000)	(107,866,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	(5,486,000)	(36,237,000)	(10,353,000)	(114,189,000)
Other	(1,523,000)	(1,825,000)	(3,105,000)	(7,462,000)
Ending Balance	624,553,000	753,396,000	624,553,000	753,396,000
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	60,443,000	168,551,000	90,055,000	171,137,000
Total Realized and Unrealized Gains Included in Earnings	27,000	3,641,000	3,780,000	6,023,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(353,000)	(71,000)	(1,154,000)	(1,272,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	10,908,000
Sales	(19,188,000)	(9,366,000)	(52,516,000)	(15,649,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	(11,243,000)	0	(19,550,000)
Other	85,000	20,000	849,000	(65,000)
Ending Balance	41,014,000	151,532,000	41,014,000	151,532,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(182,000)	3,231,000	3,498,000	4,577,000
Level 3 | Trading securities | Other asset-backed securities
Assets:
Beginning Balance	54,923,000	151,964,000	84,563,000	152,912,000
Total Realized and Unrealized Gains Included in Earnings	0	3,260,000	3,679,000	5,310,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Losses Included in Earnings	(353,000)	(71,000)	(1,154,000)	(1,013,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	(19,188,000)	(9,366,000)	(52,516,000)	(11,578,000)
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers in/out of Level 3	0	0	0	172,000
Other	108,000	46,000	918,000	30,000
Ending Balance	35,490,000	145,833,000	35,490,000	145,833,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(209,000)	3,189,000	3,397,000	4,294,000
Level 3 | Trading securities | Corporate securities
Assets:
Beginning Balance	5,520,000	16,587,000	5,492,000	18,225,000
Total Realized and Unrealized Gains Included in Earnings	27,000	381,000	101,000	713,000
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	0	0	10,908,000
Sales	0	0	0	(4,071,000)
Issuances	0	0	0
Settlements	0	0	0
Transfers in/out of Level 3	0	(11,243,000)	0	(19,722,000)
Other	(23,000)	(26,000)	(69,000)	(95,000)
Ending Balance	5,524,000	5,699,000	5,524,000	5,699,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ 27,000	$ 42,000	$ 101,000	$ 283,000