Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	$ 1,177	$ 1,205
Total debt securities, Fair Value	9,847	9,453
Total invested assets	11,024	10,658
Debt Securities | Tax-exempt municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	3,071	2,856
Debt Securities | Residential Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	34	44
Debt Securities | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	659	381
Debt Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	68	83
Debt Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	3,794	3,580
Debt Securities | 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	618	725
Debt Securities | 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	1,603	1,784
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	1,177	1,205
Total debt securities, Fair Value	0	0
Total invested assets	1,177	1,205
Quoted Prices in Active Markets for Identical Assets (Level 1) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | 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)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	0	0
Total debt securities, Fair Value	9,809	9,412
Total invested assets	9,809	9,412
Significant Other Observable Inputs (Level 2) | Debt Securities | Tax-exempt municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	3,058	2,840
Significant Other Observable Inputs (Level 2) | Debt Securities | Residential Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	34	44
Significant Other Observable Inputs (Level 2) | Debt Securities | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	659	381
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	67	82
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	3,770	3,556
Significant Other Observable Inputs (Level 2) | Debt Securities | 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	618	725
Significant Other Observable Inputs (Level 2) | Debt Securities | 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	1,603	1,784
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	0	0
Total debt securities, Fair Value	38	41
Total invested assets	38	41
Significant Unobservable Inputs (Level 3) | Debt Securities | Tax-exempt municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	13	16
Significant Unobservable Inputs (Level 3) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | 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) | Debt Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	24	24
Significant Unobservable Inputs (Level 3) | Debt Securities | 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) | Debt Securities | 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