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
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			91.50%
Number of broker quotes obtained per security			2
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations			85.00%
Other long-term investments	$ 823,514,000		$ 823,514,000			$ 414,827,000
Financial instruments owned, other	68,500,000		68,500,000			63,400,000
Securities transferred into Level 3	52,100,000	$ 19,900,000	138,200,000	$ 30,900,000
Securities transferred into Level 2 from Level 3	10,800,000	47,700,000	31,400,000	66,200,000
Transfer from level 2 to level 1	0	0	0	0
Transfers from level 1 into level 2	0	0	0	0
Interest support, YRT premium support and portfolio maintenance agreement
Determination of fair values
Other long-term investments	103,200,000		103,200,000
Golden Gate II | Interest Support Agreement
Determination of fair values
Other long-term investments	49,500,000		49,500,000
Payments made under agreement	1,000,000.0		1,000,000.0
Amount of collateralized support agreement obligations by PLC	4,900,000		4,900,000
Golden Gate II | YRT premium support agreement
Determination of fair values
Other long-term investments	50,900,000		50,900,000
Payments made under agreement			700,000
Golden Gate, Golden Gate V and West Coast Life | Portfolio maintenance agreements
Determination of fair values
Other long-term investments	2,800,000		2,800,000
Payments made under agreement			0
Level 3
Determination of fair values
Level 3 fair value	1,139,111,000	$ 538,836,000	1,139,111,000	$ 538,836,000	$ 914,620,000	514,246,000	$ 537,106,000	$ 681,034,000
Financial instruments valued in broker quotes	78,300,000		78,300,000			39,700,000
Level 3 | Annuity account
Determination of fair values
Level 3 fair value	71,800,000		71,800,000
Asset-backed securities | Level 2
Determination of fair values
Fair value	10,000,000,000.0		10,000,000,000.0
Asset-backed securities | Level 3
Determination of fair values
Fair value	487,200,000		$ 487,200,000
Other asset-backed securities
Determination of fair values
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program			100.00%
Other asset-backed securities | Level 3
Determination of fair values
Fair value	415,950,000		$ 415,950,000			421,458,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program			97.00%
Financial instruments valued in broker quotes	71,300,000		$ 71,300,000			26,200,000
Other asset-backed securities | Level 3 | Available-for-sale securities
Determination of fair values
Fair value	425,700,000		425,700,000
Other asset-backed securities | Level 3 | Trading securities
Determination of fair values
Fair value	61,500,000		61,500,000
Corporate Bonds and Securities | Level 2
Determination of fair values
Fair value	53,700,000,000		53,700,000,000
Corporate Bonds and Securities | Level 3
Determination of fair values
Fair value	1,300,000,000		1,300,000,000
Equity securities
Determination of fair values
Fair value	68,500,000		68,500,000
Federal Home Loan Bank stock	68,500,000		68,500,000
Embedded derivative - GLWB | Level 3
Determination of fair values
Fair value	217,757,000		$ 217,757,000
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)	3,500,000,000		$ 3,500,000,000
Embedded derivative - Modified coinsurance agreements | Trading Securities [Member]
Determination of fair values
Fair value	285,900,000		285,900,000
Funds withheld derivative | Level 2
Determination of fair values
Other liabilities	93,700,000		93,700,000
Corporate securities | Level 3
Determination of fair values
Fair value	1,325,112,000		1,325,112,000			631,068,000
Financial instruments valued in broker quotes	$ 7,000,000.0		$ 7,000,000.0			$ 13,500,000