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FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) quote primary_source input_method	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Determination of fair values
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	93.50%
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	84.10%
Other long-term investments	$ 561,937,000		$ 414,827,000
Financial instruments owned, other	63,400,000		63,400,000
Securities transferred into Level 3	36,000,000	$ 0
Securities transferred into Level 2 from Level 3	19,000,000	0
Transfer from level 2 to level 1	0	0
Transfers from level 1 into level 2	0	0
Interest support, YRT premium support and portfolio maintenance agreement
Determination of fair values
Other long-term investments	88,400,000
Golden Gate II \| Interest Support Agreement
Determination of fair values
Other long-term investments	35,000,000
Payments made under agreement	0
Amount of collateralized support agreement obligations by PLC	4,900,000
Golden Gate II \| YRT premium support agreement
Determination of fair values
Other long-term investments	51,000,000
Payments made under agreement	0
Golden Gate V and West Coast Life \| Portfolio maintenance agreements
Determination of fair values
Other long-term investments	2,400,000
Payments made under agreement	0
Level 3
Determination of fair values
Level 3 fair value	681,268,000	$ 569,995,000	514,246,000	$ 681,034,000
Financial instruments valued in broker quotes	71,900,000		39,700,000
Level 3 \| Annuity account
Determination of fair values
Level 3 fair value	74,600,000
Asset-backed securities \| Level 2
Determination of fair values
Fair value	7,700,000,000
Asset-backed securities \| Level 3
Determination of fair values
Fair value	$ 486,600,000
Other asset-backed securities
Determination of fair values
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Other asset-backed securities \| Level 3
Determination of fair values
Fair value	$ 419,915,000		421,458,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Financial instruments valued in broker quotes	$ 66,700,000		26,200,000
Other asset-backed securities \| Level 3 \| Available-for-sale securities
Determination of fair values
Fair value	420,100,000
Other asset-backed securities \| Level 3 \| Trading securities
Determination of fair values
Fair value	66,500,000
Corporate Bonds and Securities \| Level 2
Determination of fair values
Fair value	43,700,000,000
Corporate Bonds and Securities \| Level 3
Determination of fair values
Fair value	653,900,000
Equity securities
Determination of fair values
Fair value	63,400,000
Federal Home Loan Bank stock	63,400,000
Embedded derivative - GLWB \| Level 3
Determination of fair values
Fair value	$ 76,694,000
Embedded derivative - GLWB \| Level 3 \| Minimum
Determination of fair values
Mortality rate	87.00%
Embedded derivative - GLWB \| Level 3 \| Maximum
Determination of fair values
Mortality rate	100.00%
Embedded derivative - FIA \| Level 3 \| Minimum
Determination of fair values
Mortality rate	87.00%
Embedded derivative - FIA \| Level 3 \| Maximum
Determination of fair values
Mortality rate	100.00%
Embedded derivative - IUL \| Level 3 \| Minimum
Determination of fair values
Mortality rate	37.00%
Embedded derivative - IUL \| Level 3 \| Maximum
Determination of fair values
Mortality rate	577.00%
Embedded derivative - Modified coinsurance agreements
Determination of fair values
Statutory policy liabilities (net of policy loans)	$ 2,300,000,000
Embedded derivative - Modified coinsurance agreements \| Trading securities
Determination of fair values
Fair value	113,000,000
Funds withheld derivative \| Level 2
Determination of fair values
Other liabilities	101,300,000
Corporate securities \| Level 3
Determination of fair values
Fair value	648,608,000		631,068,000
Financial instruments valued in broker quotes	$ 5,200,000		$ 13,500,000