Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	$ 9,110	$ 9,547
Measured on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	2,229	1,712
Total debt securities, Fair Value	9,110	9,547
Total invested assets	11,339	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	2,668	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	13	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	985	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	263	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	2,958	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	332	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	1,891	1,498
Quoted Prices in Active Markets (Level 1) | Measured on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	2,229	1,712
Total debt securities, Fair Value	0	0
Total invested assets	2,229	1,712
Quoted Prices in Active Markets (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 (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 (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 (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 (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 (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 (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
Other Observable Inputs (Level 2) | Measured on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	0	0
Total debt securities, Fair Value	9,099	9,515
Total invested assets	9,099	9,515
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	2,663	3,060
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	13	17
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	985	843
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	263	28
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	2,952	3,695
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	332	374
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	1,891	1,498
Unobservable Inputs (Level 3) | Measured on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents, Fair Value	0	0
Total debt securities, Fair Value	11	32
Total invested assets	11	32
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	5	8
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
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
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	0	1
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	6	23
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
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