INVESTMENT OPERATIONS - Schedule of Available-for-Sale Credit Losses on Fixed Maturities with Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		$ 0
Additions for securities for which allowance was not previously recorded	$ 125,470	125,470
Ending balance	22,742	22,742
Corporate securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		0
Additions for securities for which allowance was not previously recorded		62,442
Adjustments on previously recorded allowances due to change in expected cash flows		19,887
Reductions on previously recorded allowances due to disposal of security in the current period		(969)
Write-offs of previously recorded allowances due to intent or requirement to sell		(63,618)
Ending balance	17,742	17,742
Commercial mortgage-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		0
Additions for securities for which allowance was not previously recorded		3,598
Adjustments on previously recorded allowances due to change in expected cash flows		0
Reductions on previously recorded allowances due to disposal of security in the current period		0
Write-offs of previously recorded allowances due to intent or requirement to sell		0
Ending balance	3,598	3,598
Other asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		0
Additions for securities for which allowance was not previously recorded		658
Adjustments on previously recorded allowances due to change in expected cash flows		770
Reductions on previously recorded allowances due to disposal of security in the current period		(26)
Write-offs of previously recorded allowances due to intent or requirement to sell		0
Ending balance	1,402	1,402
Total fixed maturity securities - available-for-sale
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		0
Additions for securities for which allowance was not previously recorded		66,698
Adjustments on previously recorded allowances due to change in expected cash flows		20,657
Reductions on previously recorded allowances due to disposal of security in the current period		(995)
Write-offs of previously recorded allowances due to intent or requirement to sell		(63,618)
Ending balance	$ 22,742	$ 22,742