Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	$ 9,572	$ 9,547
Measured On Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,779	1,712
Total debt securities, Fair Value	9,572	9,547
Total invested assets	11,351	11,259
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	2,756	3,068
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	16	17
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	891	843
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	139	29
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	3,646	3,718
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	366	374
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	1,758	1,498
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	1,779	1,712
Total debt securities, Fair Value	0	0
Total invested assets	1,779	1,712
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	9,560	9,515
Total invested assets	9,560	9,515
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	2,750	3,060
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	16	17
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	891	843
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	138	28
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	3,641	3,695
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	366	374
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	1,758	1,498
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	12	32
Total invested assets	12	32
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	6	8
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	1
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	23
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