Financial Assets Measured At Fair Value On Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	$ 897	$ 1,205
Total debt securities, Fair Value	9,895	9,453
Total invested assets	10,792	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	2,878	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	36	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	658	381
Debt Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	37	83
Debt Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	3,781	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	563	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	1,942	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	897	1,205
Total debt securities, Fair Value	0	0
Total invested assets	897	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	9,857	9,412
Total invested assets	9,857	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	2,865	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	36	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	658	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	36	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	3,757	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	563	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	1,942	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