FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets and liabilities measured at fair value on a recurring basis
Minimum percentage of the Company's fixed maturity securities priced by third party pricing services	91.00%
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	$ 65,700	$ 65,900
Level 3
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	821,441	659,459	$ 788,312	$ 1,028,128	$ 1,060,904	$ 678,068
Financial instruments that are valued using broker quotes	50,600	128,200
Level 3 | Annuity account
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	85,028	$ 87,616	$ 86,094	$ 88,857	$ 88,820	$ 92,512
Asset-backed securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair Value	5,300,000
Asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value	$ 588,600
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	$ 553,005	$ 553,308
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	$ 35,600	93,900
Other asset-backed securities | Level 3 | Available-for-sale securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	553,100
Other asset-backed securities | Level 3 | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	35,500
Corporate Bonds And Securities | Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair Value	33,200,000
Corporate Bonds And Securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value	630,100
Equity securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	66,200
Federal Home Loan Bank (FHLB) Stock	65,500
Equity securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	$ 500	3,100
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)	$ 2,400,000
Embedded derivative - Modified coinsurance agreements | Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair Value	218,800
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value	615,576	638,279
Financial instruments that are valued using broker quotes	$ 14,500	$ 31,300