FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) quote primary_source input_method	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 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			93.30%
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	$ 63,600,000		$ 63,600,000		$ 65,700,000
Transfers into Level 3	19,900,000	$ 0	30,900,000	$ 0
Securities transferred from level 3 to level 2	47,700,000	17,400,000	66,200,000	28,900,000
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	632,943,000	821,441,000	632,943,000	821,441,000	844,362,000	$ 653,014,000	$ 788,312,000	$ 659,459,000
Financial instruments that are valued using broker quotes	60,500,000		60,500,000		50,400,000
Level 3 | Annuity account
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	78,463,000	$ 85,028,000	78,463,000	$ 85,028,000	$ 83,472,000	$ 80,098,000	$ 86,094,000	$ 87,616,000
Asset-backed securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	7,100,000,000		7,100,000,000
Asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	549,700,000		$ 549,700,000
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	502,020,000		$ 502,020,000		$ 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	47,700,000		$ 47,700,000		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	521,700,000		521,700,000
Other asset-backed securities | Level 3 | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	28,000,000		28,000,000
Corporate Bonds And Securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	43,100,000,000		43,100,000,000
Corporate Bonds And Securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	633,500,000		633,500,000
Equity securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	64,300,000		64,300,000
Federal Home Loan Bank (FHLB) stock	63,400,000		63,400,000
Equity securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	700,000		$ 700,000		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)			87.00%
Embedded derivative - FIA | Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			100.00%
Embedded derivative - IUL | Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			37.00%
Embedded derivative - IUL | Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			577.00%
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	60,000,000		60,000,000
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	621,392,000		621,392,000		617,770,000
Financial instruments that are valued using broker quotes	$ 12,100,000		$ 12,100,000		$ 14,600,000