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Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | $ 9,678 | $ 9,110
Measured On Recurring Basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 2,669 | 2,229
Total debt securities, Fair Value | 9,678 | 9,110
Total invested assets | 12,347 | 11,339
Measured On Recurring Basis | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 3,038 | 2,668
Measured On Recurring Basis | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 11 | 13
Measured On Recurring Basis | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 859 | 985
Measured On Recurring Basis | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 253 | 263
Measured On Recurring Basis | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 3,030 | 2,958
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] |  |
Total debt securities, Fair Value | 453 | 332
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] |  |
Total debt securities, Fair Value | 2,034 | 1,891
Quoted Prices in Active Markets for Identical Assets (Level 1) | Measured On Recurring Basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 2,669 | 2,229
Total debt securities, Fair Value | 0 | 0
Total invested assets | 2,669 | 2,229
Quoted Prices in Active Markets for Identical Assets (Level 1) | Measured On Recurring Basis | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Measured On Recurring Basis | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Measured On Recurring Basis | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Measured On Recurring Basis | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Measured On Recurring Basis | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | 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] |  |
Total debt securities, Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | 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] |  |
Total debt securities, Fair Value | 0 | 0
Significant Other Observable Inputs (Level 2) | Measured On Recurring Basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 0 | 0
Total debt securities, Fair Value | 9,669 | 9,099
Total invested assets | 9,669 | 9,099
Significant Other Observable Inputs (Level 2) | Measured On Recurring Basis | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 3,035 | 2,663
Significant Other Observable Inputs (Level 2) | Measured On Recurring Basis | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 11 | 13
Significant Other Observable Inputs (Level 2) | Measured On Recurring Basis | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 859 | 985
Significant Other Observable Inputs (Level 2) | Measured On Recurring Basis | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 252 | 263
Significant Other Observable Inputs (Level 2) | Measured On Recurring Basis | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 3,025 | 2,952
Significant Other Observable Inputs (Level 2) | 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] |  |
Total debt securities, Fair Value | 453 | 332
Significant Other Observable Inputs (Level 2) | 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] |  |
Total debt securities, Fair Value | 2,034 | 1,891
Significant Unobservable Inputs (Level 3) | Measured On Recurring Basis |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 0 | 0
Total debt securities, Fair Value | 9 | 11
Total invested assets |  | 11
Significant Unobservable Inputs (Level 3) | Measured On Recurring Basis | Tax-exempt municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 3 | 5
Significant Unobservable Inputs (Level 3) | Measured On Recurring Basis | Residential Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Significant Unobservable Inputs (Level 3) | Measured On Recurring Basis | Commercial Mortgage-Backed Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 0 | 0
Significant Unobservable Inputs (Level 3) | Measured On Recurring Basis | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 1 | 0
Significant Unobservable Inputs (Level 3) | Measured On Recurring Basis | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total debt securities, Fair Value | 5 | 6
Significant Unobservable Inputs (Level 3) | 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] |  |
Total debt securities, Fair Value | 0 | 0
Significant Unobservable Inputs (Level 3) | 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] |  |
Total debt securities, Fair Value | $ 0 | $ 0

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