FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
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			91.70%
Number of broker quotes obtained per security			2
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations			84.30%
Other long-term investments	$ 676,456,000		$ 676,456,000			$ 414,827,000
Financial instruments owned, other	68,500,000		68,500,000			63,400,000
Securities transferred into Level 3	50,100,000	$ 11,100,000	86,100,000	$ 11,100,000
Securities transferred into Level 2 from Level 3	1,600,000	18,600,000	20,600,000	18,600,000
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	95,600,000		95,600,000
Golden Gate II | Interest Support Agreement
Determination of fair values
Other long-term investments	42,300,000		42,300,000
Payments made under agreement	1,000,000		1,000,000
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	50,800,000		50,800,000
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	2,500,000		2,500,000
Payments made under agreement			0
Level 3
Determination of fair values
Level 3 fair value	914,620,000	$ 537,106,000	914,620,000	$ 537,106,000	$ 681,268,000	514,246,000	$ 569,995,000	$ 681,034,000
Financial instruments valued in broker quotes	77,000,000		77,000,000			39,700,000
Level 3 | Annuity account
Determination of fair values
Level 3 fair value	72,600,000		72,600,000
Asset-backed securities | Level 2
Determination of fair values
Fair value	9,200,000,000		9,200,000,000
Asset-backed securities | Level 3
Determination of fair values
Fair value	489,900,000		$ 489,900,000
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	418,178,000		$ 418,178,000			421,458,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program			97.00%
Financial instruments valued in broker quotes	71,700,000		$ 71,700,000			26,200,000
Other asset-backed securities | Level 3 | Available-for-sale securities
Determination of fair values
Fair value	427,700,000		427,700,000
Other asset-backed securities | Level 3 | Trading securities
Determination of fair values
Fair value	62,200,000		62,200,000
Corporate Bonds and Securities | Level 2
Determination of fair values
Fair value	52,500,000,000		52,500,000,000
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	146,556,000		$ 146,556,000
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	210,500,000		210,500,000
Funds withheld derivative | Level 2
Determination of fair values
Other liabilities	91,100,000		91,100,000
Corporate securities | Level 3
Determination of fair values
Fair value	1,328,490,000		1,328,490,000			631,068,000
Financial instruments valued in broker quotes	$ 5,300,000		$ 5,300,000			$ 13,500,000