FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
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			91.60%
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	52,100,000	$ 19,900,000	138,200,000	$ 30,900,000
Securities transferred from level 3 to level 2	10,800,000	47,700,000	31,400,000	66,200,000
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	1,546,772,000	632,943,000	1,546,772,000	632,943,000	$ 1,169,662,000	706,061,000	$ 653,014,000	$ 844,362,000
Financial instruments that are valued using broker quotes	79,400,000		79,400,000			40,400,000
Level 3 | Annuity account
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	71,820,000	$ 78,463,000	71,820,000	$ 78,463,000	$ 72,585,000	76,119,000	$ 80,098,000	$ 83,472,000
Asset-backed securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	10,100,000,000		10,100,000,000
Asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	487,200,000		$ 487,200,000
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	415,950,000		$ 415,950,000			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	71,300,000		$ 71,300,000			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	425,700,000		425,700,000
Other asset-backed securities | Level 3 | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	61,500,000		61,500,000
Corporate bonds and securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	53,900,000,000		53,900,000,000
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	69,900,000		69,900,000
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	1,100,000		$ 1,100,000			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	285,900,000		285,900,000
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	1,325,112,000		1,325,112,000			631,068,000
Financial instruments that are valued using broker quotes	$ 7,000,000.0		$ 7,000,000.0			$ 13,500,000