FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Assets and liabilities measured at fair value on a recurring basis
Number of sources of information considered			1
Number of sources of information			3
Minimum percentage of the Company's fixed maturity securities priced by third party pricing services			91.80%
Number of broker quotes obtained per security			2
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	$ 68,700,000		$ 68,700,000			$ 63,600,000
Transfers into Level 3	50,100,000	$ 11,100,000	86,100,000	$ 11,100,000
Securities transferred from level 3 to level 2	1,600,000	18,600,000	20,600,000	18,600,000
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	1,169,662,000	653,014,000	1,169,662,000	653,014,000	$ 872,504,000	706,061,000	$ 702,501,000	$ 844,362,000
Financial instruments that are valued using broker quotes	77,900,000		77,900,000			40,400,000
Level 3 | Annuity account
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	72,585,000	$ 80,098,000	72,585,000	$ 80,098,000	$ 74,613,000	76,119,000	$ 81,399,000	$ 83,472,000
Asset-backed securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	9,200,000,000		9,200,000,000
Asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	489,900,000		$ 489,900,000
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%
Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	418,178,000		$ 418,178,000			421,458,000
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	71,700,000		$ 71,700,000			26,200,000
Other asset-backed securities | Level 3 | Available-for-sale securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	427,700,000		427,700,000
Other asset-backed securities | Level 3 | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	62,200,000		62,200,000
Corporate bonds and securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	52,700,000,000		52,700,000,000
Corporate bonds and securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	1,300,000,000		1,300,000,000
Equity securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	69,700,000		69,700,000
Federal Home Loan Bank (FHLB) stock	68,500,000		68,500,000
Equity securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	900,000		$ 900,000			700,000
Embedded derivative - GLWB | Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			87.00%
Embedded derivative - GLWB | Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)			100.00%
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)	3,500,000,000		$ 3,500,000,000
Embedded derivative - Modified coinsurance agreements | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	210,500,000		210,500,000
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	1,328,490,000		1,328,490,000			631,068,000
Financial instruments that are valued using broker quotes	$ 5,300,000		$ 5,300,000			$ 13,500,000