Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt securities, Fair Value	$ 9,302	$ 9,547
Measured On Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,459	1,712
Total debt securities, Fair Value	9,302	9,547
Total invested assets	10,761	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	2,751	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	1,031	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	273	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	3,339	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	328	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	1,567	1,498
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	1,459	1,712
Total debt securities, Fair Value	0	0
Total invested assets	1,459	1,712
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	9,291	9,515
Total invested assets	9,291	9,515
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	2,746	3,060
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	13	17
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	1,031	843
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	272	28
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	3,334	3,695
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	328	374
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	1,567	1,498
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	11	32
Total invested assets		32
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	$ 5	8
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
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	5	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