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