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FAIR VALUE - Financial Assets Measured on Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities $ 17,077 $ 17,008
U.S. Treasury and agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 2,844 2,667
Mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 3,439 3,522
Tax-exempt municipal securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 826 858
Residential    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 394 400
Commercial    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 1,308 1,345
Asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 1,517 1,771
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 6,749 6,445
Recurring Basis    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 5,594 4,582
Total debt securities 17,077 17,008
Total invested assets 22,671 21,590
Recurring Basis | U.S. Treasury and agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 2,844 2,667
Recurring Basis | Mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 3,439 3,522
Recurring Basis | Tax-exempt municipal securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 826 858
Recurring Basis | Residential    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 394 400
Recurring Basis | Commercial    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 1,308 1,345
Recurring Basis | Asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 1,517 1,771
Recurring Basis | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 6,749 6,445
Recurring Basis | Quoted Prices in Active Markets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 5,594 4,582
Total debt securities 0 0
Total invested assets 5,594 4,582
Recurring Basis | Quoted Prices in Active Markets (Level 1) | U.S. Treasury and agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Quoted Prices in Active Markets (Level 1) | Mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Quoted Prices in Active Markets (Level 1) | Tax-exempt municipal securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Quoted Prices in Active Markets (Level 1) | Residential    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Quoted Prices in Active Markets (Level 1) | Commercial    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Quoted Prices in Active Markets (Level 1) | Asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Quoted Prices in Active Markets (Level 1) | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Total debt securities 16,851 16,790
Total invested assets 16,851 16,790
Recurring Basis | Other Observable Inputs (Level 2) | U.S. Treasury and agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 2,844 2,667
Recurring Basis | Other Observable Inputs (Level 2) | Mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 3,439 3,522
Recurring Basis | Other Observable Inputs (Level 2) | Tax-exempt municipal securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 826 858
Recurring Basis | Other Observable Inputs (Level 2) | Residential    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 391 396
Recurring Basis | Other Observable Inputs (Level 2) | Commercial    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 1,308 1,345
Recurring Basis | Other Observable Inputs (Level 2) | Asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 1,479 1,733
Recurring Basis | Other Observable Inputs (Level 2) | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 6,564 6,269
Recurring Basis | Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Total debt securities 226 218
Total invested assets 226 218
Recurring Basis | Unobservable Inputs (Level 3) | U.S. Treasury and agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Unobservable Inputs (Level 3) | Mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Unobservable Inputs (Level 3) | Tax-exempt municipal securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Unobservable Inputs (Level 3) | Residential    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 3 4
Recurring Basis | Unobservable Inputs (Level 3) | Commercial    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 0 0
Recurring Basis | Unobservable Inputs (Level 3) | Asset-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities 38 38
Recurring Basis | Unobservable Inputs (Level 3) | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total debt securities $ 185 $ 176