Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	$ 10,035	$ 9,800
Measured On Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,606	876
Total debt securities, Fair Value	10,035	9,800
Total invested assets	12,641	10,676
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	3,151	2,971
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	18	22
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	696	673
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	41	63
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	3,708	3,667
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	852	584
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	1,569	1,820
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	2,606	876
Total debt securities, Fair Value	0	0
Total invested assets	2,606	876
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	9,997	9,763
Total invested assets	9,997	9,763
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	3,138	2,958
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	18	22
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	696	673
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	40	62
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	3,684	3,644
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	852	584
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	1,569	1,820
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	38	37
Total invested assets	38	37
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	13	13
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	24	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