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FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Determination of fair values
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	92.40%
Number of broker quotes obtained per security	2
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations	86.20%
Other long-term investments	$ 981,739,000		$ 995,493,000
Financial instruments owned, other	73,200,000		73,000,000.0
Securities transferred into Level 3	29,500,000	$ 36,000,000.0
Securities transferred into Level 2 from Level 3		19,000,000.0
Interest support, YRT premium support and portfolio maintenance agreement
Determination of fair values
Other long-term investments	109,400,000
Golden Gate II \| Interest Support Agreement
Determination of fair values
Other long-term investments	55,700,000
Payments made under agreement	2,400,000
Amount of collateralized support agreement obligations by PLC	5,500,000
Golden Gate II \| YRT premium support agreement
Determination of fair values
Other long-term investments	52,200,000
Payments made under agreement	1,600,000
Golden Gate, Golden Gate V and West Coast Life \| Portfolio maintenance agreements
Determination of fair values
Other long-term investments	1,400,000
Payments made under agreement	0
Level 3
Determination of fair values
Level 3 fair value	1,330,669,000	$ 681,268,000	1,087,700,000	$ 514,246,000
Financial instruments valued in broker quotes	98,700,000		76,800,000
Asset-backed securities \| Level 2
Determination of fair values
Fair value	11,100,000,000
Asset-backed securities \| Level 3
Determination of fair values
Fair value	$ 512,100,000
Other asset-backed securities
Determination of fair values
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	9700.00%
Other asset-backed securities \| Level 3
Determination of fair values
Fair value	$ 436,329,000		421,219,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Financial instruments valued in broker quotes	$ 85,700,000		65,400,000
Other asset-backed securities \| Level 3 \| Available-for-sale securities
Determination of fair values
Fair value	447,100,000
Other asset-backed securities \| Level 3 \| Trading securities
Determination of fair values
Fair value	65,000,000.0
Corporate Bonds and Securities \| Level 2
Determination of fair values
Fair value	50,200,000,000
Corporate Bonds and Securities \| Level 3
Determination of fair values
Fair value	1,300,000,000
Equity securities
Determination of fair values
Fair value	73,000,000.0
Federal Home Loan Bank stock	73,000,000.0
Embedded derivative - GLWB \| Level 3
Determination of fair values
Fair value	$ 596,619,000
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	156.00%
Funds withheld derivative \| Level 2
Determination of fair values
Other liabilities	$ 228,000,000.0
Corporate securities \| Level 3
Determination of fair values
Fair value	1,279,769,000		1,373,714,000
Financial instruments valued in broker quotes	$ 13,000,000.0		$ 11,400,000