FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) quote primary_source input_method	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Assets and liabilities measured at fair value on a recurring basis
Number of sources of information considered | primary_source			1
Number of sources of information | input_method			3
Minimum percentage of the Company's fixed maturity securities priced by third party pricing services			93.40%
Number of broker quotes obtained per security | quote			1
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations			100.00%
Financial instruments with book value approximating fair value	$ 87,500,000		$ 87,500,000		$ 65,700,000
Transfers into Level 3	11,100,000	$ 0	11,100,000	$ 0
Securities transferred from level 3 to level 2	18,600,000	2,200,000	18,600,000	11,500,000
Transfers from Level 2 to Level 1	0	0	0	0
Transfers from Level 1	0	0	0	0
Level 3
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	653,014,000	788,312,000	653,014,000	788,312,000	844,362,000	$ 702,501,000	$ 673,489,000	$ 659,459,000
Financial instruments that are valued using broker quotes	85,400,000		85,400,000		50,400,000
Level 3 | Annuity account
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	80,098,000	$ 86,094,000	80,098,000	$ 86,094,000	$ 83,472,000	$ 81,399,000	$ 86,415,000	$ 87,616,000
Other asset-backed securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	6,800,000,000		6,800,000,000
Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	609,600,000		$ 609,600,000
Other asset-backed securities
Assets and liabilities measured at fair value on a recurring basis
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (FFELP), minimum			97.00%		97.00%
Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	515,379,000		$ 515,379,000		$ 504,228,000
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (FFELP), minimum			97.00%
Financial instruments that are valued using broker quotes	72,400,000		$ 72,400,000		35,400,000
Other asset-backed securities | Level 3 | Available-for-sale securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	584,700,000		584,700,000
Other asset-backed securities | Level 3 | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	24,900,000		24,900,000
Corporate Bonds And Securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	42,600,000,000		42,600,000,000
Corporate Bonds And Securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	650,100,000		650,100,000
Equity securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	66,100,000		66,100,000
Federal Home Loan Bank (FHLB) stock	65,500,000		65,500,000
Equity securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	400,000		$ 400,000		400,000
Embedded derivative - GLWB | Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			91.10%
Embedded derivative - GLWB | Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			106.60%
Embedded derivative - FIA | Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			46.00%
Embedded derivative - FIA | Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			113.00%
Embedded derivative - IUL | Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			34.00%
Embedded derivative - IUL | Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			152.00%
Embedded derivative - Modified coinsurance agreements
Assets and liabilities measured at fair value on a recurring basis
Statutory policy liabilities (net of policy loans)	2,300,000,000		$ 2,300,000,000
Embedded derivative - Modified coinsurance agreements | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	76,800,000		76,800,000
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	637,479,000		637,479,000		617,770,000
Financial instruments that are valued using broker quotes	$ 12,600,000		$ 12,600,000		$ 14,600,000