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FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 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			92.60%
Number of broker quotes obtained per security			2
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations			87.40%
Other long-term investments	$ 1,407,774,000		$ 1,407,774,000			$ 995,493,000
Financial instruments owned, other	80,300,000		80,300,000			73,000,000.0
Securities transferred into Level 3	31,200,000	$ 52,100,000	115,500,000	$ 138,200,000
Securities transferred into Level 2 from Level 3		10,800,000	1,200,000	31,400,000
Interest support, YRT premium support and portfolio maintenance agreement
Determination of fair values
Other long-term investments	133,500,000		133,500,000
Golden Gate II \| Interest Support Agreement
Determination of fair values
Other long-term investments	78,600,000		78,600,000
Payments made under agreement			2,400,000
Amount of collateralized support agreement obligations by PLC	5,500,000		5,500,000
Golden Gate II \| YRT premium support agreement
Determination of fair values
Other long-term investments	53,200,000		53,200,000
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	1,700,000		1,700,000
Payments made under agreement			0
Level 3
Determination of fair values
Level 3 fair value	1,786,496,000	$ 1,139,111,000	1,786,496,000	$ 1,139,111,000	$ 1,732,315,000	1,087,700,000	$ 914,620,000	$ 514,246,000
Financial instruments valued in broker quotes	98,100,000		98,100,000			76,800,000
Asset-backed securities \| Level 2
Determination of fair values
Fair value	11,000,000,000.0		11,000,000,000.0
Asset-backed securities \| Level 3
Determination of fair values
Fair value	507,600,000		$ 507,600,000
Other asset-backed securities
Determination of fair values
Underlying collateral percentage			9700.00%
Other asset-backed securities \| Level 3
Determination of fair values
Fair value	434,496,000		$ 434,496,000			421,219,000
Underlying collateral percentage			97.00%
Financial instruments valued in broker quotes	79,300,000		$ 79,300,000			65,400,000
Other asset-backed securities \| Level 3 \| Available-for-sale securities
Determination of fair values
Fair value	445,200,000		445,200,000
Other asset-backed securities \| Level 3 \| Trading securities
Determination of fair values
Fair value	62,400,000		62,400,000
Corporate bonds and securities \| Level 2
Determination of fair values
Fair value	56,500,000,000		56,500,000,000
Corporate bonds and securities \| Level 3
Determination of fair values
Fair value	1,400,000,000		1,400,000,000
Equity securities
Determination of fair values
Fair value	86,300,000		86,300,000
Federal Home Loan Bank stock	80,300,000		80,300,000
Equity securities \| Level 3
Determination of fair values
Financial instruments valued in broker quotes	6,000,000.0		6,000,000.0
Embedded derivative - GLWB \| Level 3
Determination of fair values
Fair value	518,246,000		$ 518,246,000
Embedded derivative - GLWB \| Level 3 \| Minimum
Determination of fair values
Mortality rate			88.00%
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			88.00%
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			36.00%
Embedded derivative - IUL \| Level 3 \| Maximum
Determination of fair values
Mortality rate			161.00%
Funds withheld derivative \| Level 2
Determination of fair values
Other liabilities	145,000,000.0		$ 145,000,000.0
Corporate securities \| Level 3
Determination of fair values
Fair value	1,357,544,000		1,357,544,000			1,373,714,000
Financial instruments valued in broker quotes	$ 12,800,000		$ 12,800,000			$ 11,400,000