FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of Beginning and Ending Balances for Fair Value Measurements (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Transfers
Transfers into Level 3	$ 0	$ 0
Securities transferred from level 3 to level 2	0	9,300,000
Transfers from Level 2 to Level 1	0	0
Transfers from Level 1	0	0
Level 3
Assets:
Beginning Balance	1,374,044,000	1,510,043,000
Total Realized and Unrealized Gains Included in Earnings	9,058,000	14,569,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	1,913,000	11,303,000
Total Realized and Unrealized Losses Included in Earnings	(687,000)	(5,174,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,735,000)	(1,113,000)
Purchases	35,000,000	37,259,000
Sales	(24,045,000)	(60,210,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(9,287,000)
Other	(655,000)	319,000
Ending Balance	1,380,893,000	1,497,709,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	8,371,000	12,026,000
Liabilities:
Beginning Balance	844,362,000	659,459,000
Total Realized and Unrealized Gains Included in Earnings	161,318,000	44,263,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(22,324,000)	(60,381,000)
Total Realized and Unrealized Losses Included In Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	441,000	180,000
Settlements	3,308,000	2,268,000
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	702,501,000	673,489,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	139,788,000	(15,231,000)
Level 3 | Annuity account
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	(794,000)	(887,000)
Total Realized and Unrealized Losses Included In Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	441,000	180,000
Settlements	3,308,000	2,268,000
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	81,399,000	86,415,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Other liabilities
Liabilities:
Beginning Balance	760,890,000	571,843,000
Total Realized and Unrealized Gains Included in Earnings	161,318,000	44,263,000
Total Realized and Unrealized Gains Included In Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(21,530,000)	(59,494,000)
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	621,102,000	587,074,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	139,788,000	(15,231,000)
Level 3 | Total investments
Assets:
Beginning Balance	1,374,044,000	1,510,043,000
Total Realized and Unrealized Gains Included in Earnings	9,058,000	14,569,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	1,913,000	11,303,000
Total Realized and Unrealized Losses Included in Earnings	(687,000)	(5,174,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,735,000)	(1,113,000)
Purchases	35,000,000	37,259,000
Sales	(24,045,000)	(60,210,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(9,287,000)
Other	(655,000)	319,000
Ending Balance	1,380,893,000	1,497,709,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	8,371,000	12,026,000
Level 3 | Fixed maturities
Assets:
Beginning Balance	1,171,930,000	1,316,708,000
Total Realized and Unrealized Gains Included in Earnings	194,000	3,508,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	1,913,000	11,301,000
Total Realized and Unrealized Losses Included in Earnings	(122,000)	(586,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,735,000)	(1,113,000)
Purchases	35,000,000	37,259,000
Sales	(24,045,000)	(60,210,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(9,290,000)
Other	(655,000)	320,000
Ending Balance	1,170,480,000	1,297,897,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	72,000	2,922,000
Level 3 | Equity securities
Assets:
Beginning Balance	66,110,000	69,010,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	2,000
Total Realized and Unrealized Losses Included in Earnings	(49,000)	(2,630,000)
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	3,000
Other	0	(1,000)
Ending Balance	66,061,000	66,384,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	(49,000)	1,000
Level 3 | Other long-term investments
Assets:
Beginning Balance	136,004,000	124,325,000
Total Realized and Unrealized Gains Included in Earnings	8,864,000	11,061,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(516,000)	(1,958,000)
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	144,352,000	133,428,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	8,348,000	9,103,000
Level 3 | Available-for-sale securities | Fixed maturities
Assets:
Beginning Balance	1,131,266,000	1,226,653,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	1,913,000	11,301,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,735,000)	(1,113,000)
Purchases	35,000,000	37,259,000
Sales	(23,649,000)	(40,902,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(9,290,000)
Other	(597,000)	(267,000)
Ending Balance	1,130,198,000	1,223,641,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Available-for-sale securities | Residential mortgage-backed 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	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	(3,000)
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 | Available-for-sale securities | Commercial mortgage-backed 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 | Available-for-sale securities | Other asset-backed securities
Assets:
Beginning Balance	504,365,000	562,604,000
Total Realized and Unrealized Gains Included in Earnings	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	514,000	3,530,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(1,634,000)	(831,000)
Purchases	0	0
Sales	(14,000)	(2,015,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(6,643,000)
Other	558,000	291,000
Ending Balance	503,789,000	556,936,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Available-for-sale securities | Corporate 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	1,399,000	7,771,000
Total Realized and Unrealized Losses Included in Earnings	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(12,101,000)	(282,000)
Purchases	35,000,000	37,259,000
Sales	(23,635,000)	(38,884,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	(2,647,000)
Other	(1,155,000)	(558,000)
Ending Balance	626,409,000	666,705,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	0	0
Level 3 | Trading securities | Fixed maturities
Assets:
Beginning Balance	40,664,000	90,055,000
Total Realized and Unrealized Gains Included in Earnings	194,000	3,508,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(122,000)	(586,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(396,000)	(19,308,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	(58,000)	587,000
Ending Balance	40,282,000	74,256,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	72,000	2,922,000
Level 3 | Trading securities | Other asset-backed securities
Assets:
Beginning Balance	35,222,000	84,563,000
Total Realized and Unrealized Gains Included in Earnings	194,000	3,474,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(28,000)	(586,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(396,000)	(19,308,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	(34,000)	609,000
Ending Balance	34,958,000	68,752,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	166,000	2,888,000
Level 3 | Trading securities | Corporate securities
Assets:
Beginning Balance	5,442,000	5,492,000
Total Realized and Unrealized Gains Included in Earnings	0	34,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Earnings	(94,000)	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	(24,000)	(22,000)
Ending Balance	5,324,000	5,504,000
Total Gains (losses) included in Earnings related to Instruments still held at the Reporting Date	$ (94,000)	$ 34,000