INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Held-to-Maturity (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Schedule of Investments [Line Items]
Fair Value	$ 2,990,170,000	$ 3,025,790,000
Allowance for credit losses on held-to-maturity securities	0		$ 0
Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	2,680,324,000	2,823,881,000
Gross Unrecognized Holding Gains	309,846,000	201,909,000
Gross Unrecognized Holding Losses	0	0
Fair Value	2,990,170,000	3,025,790,000
Fixed maturities \| Red Mountain, LLC
Schedule of Investments [Line Items]
Amortized Cost	822,324,000	795,881,000
Gross Unrecognized Holding Gains	140,435,000	81,022,000
Gross Unrecognized Holding Losses	0	0
Fair Value	962,759,000	876,903,000
Fixed maturities \| Steel City, LLC
Schedule of Investments [Line Items]
Amortized Cost	1,858,000,000	2,028,000,000
Gross Unrecognized Holding Gains	169,411,000	120,887,000
Gross Unrecognized Holding Losses	0	0
Fair Value	$ 2,027,411,000	$ 2,148,887,000

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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