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FAIR VALUE - Financial Assets Measured on Recurring Basis (Detail) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | $ 10,785 | $ 9,798
Measured On Recurring Basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 7,786 | 3,654
Debt securities | 10,785 | 9,798
Total invested assets | 18,571 | 13,452
Measured On Recurring Basis | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 3,306 | 3,305
Measured On Recurring Basis | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 8 | 9
Measured On Recurring Basis | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 394 | 304
Measured On Recurring Basis | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 135 | 160
Measured On Recurring Basis | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 4,680 | 3,597
Measured On Recurring Basis | U.S. Treasury and other U.S. government corporations and agencies | U.S. Treasury and agency obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 716 | 786
Measured On Recurring Basis | U.S. Treasury and other U.S. government corporations and agencies | Mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 1,546 | 1,637
Measured On Recurring Basis | Quoted Prices in Active Markets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 7,786 | 3,654
Debt securities | 0 | 0
Total invested assets | 7,786 | 3,654
Measured On 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] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Quoted Prices in Active Markets (Level 1) | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Quoted Prices in Active Markets (Level 1) | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On 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] |  |
Debt securities | 0 | 0
Measured On 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] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Quoted Prices in Active Markets (Level 1) | U.S. Treasury and other U.S. government corporations and agencies | U.S. Treasury and agency obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Quoted Prices in Active Markets (Level 1) | U.S. Treasury and other U.S. government corporations and agencies | Mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 0 | 0
Debt securities | 10,781 | 9,791
Total invested assets | 10,781 | 9,791
Measured On Recurring Basis | Other Observable Inputs (Level 2) | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 3,306 | 3,302
Measured On Recurring Basis | Other Observable Inputs (Level 2) | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 8 | 9
Measured On Recurring Basis | Other Observable Inputs (Level 2) | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 394 | 304
Measured On Recurring Basis | Other Observable Inputs (Level 2) | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 135 | 160
Measured On Recurring Basis | Other Observable Inputs (Level 2) | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 4,676 | 3,593
Measured On Recurring Basis | Other Observable Inputs (Level 2) | U.S. Treasury and other U.S. government corporations and agencies | U.S. Treasury and agency obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 716 | 786
Measured On Recurring Basis | Other Observable Inputs (Level 2) | U.S. Treasury and other U.S. government corporations and agencies | Mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 1,546 | 1,637
Measured On Recurring Basis | Unobservable Inputs (Level 3) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 0 | 0
Debt securities | 4 | 7
Total invested assets | 4 | 7
Measured On Recurring Basis | Unobservable Inputs (Level 3) | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 3
Measured On Recurring Basis | Unobservable Inputs (Level 3) | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Unobservable Inputs (Level 3) | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Unobservable Inputs (Level 3) | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Unobservable Inputs (Level 3) | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 4 | 4
Measured On Recurring Basis | Unobservable Inputs (Level 3) | U.S. Treasury and other U.S. government corporations and agencies | U.S. Treasury and agency obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | 0 | 0
Measured On Recurring Basis | Unobservable Inputs (Level 3) | U.S. Treasury and other U.S. government corporations and agencies | Mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities | $ 0 | $ 0

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