Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 2,239	$ 633
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(21)	(5)
12 months or more Fair Value		47
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		(5)
Total Fair Value	2,239	680
Total Unrealized Losses	(21)	(10)
US Treasury and Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		70
12 months or more Fair Value		1
Total Fair Value		71
US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
12 months or more Fair Value		2
Total Fair Value		2
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,784	34
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(18)	(1)
12 months or more Fair Value		6
Total Fair Value	1,784	40
Total Unrealized Losses	(18)	(1)
Other Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	454	523
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(3)	(4)
12 months or more Fair Value		38
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		(5)
Total Fair Value	454	561
Total Unrealized Losses	(3)	(9)
Equity Securities, Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1	6
Total Fair Value	$ 1	$ 6