Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 3,392	$ 2,239
Less than 12 months Unrealized Losses	(39)	(21)
12 months or more Fair Value	102
12 months or more Unrealized Losses	(3)
Total Fair Value	3,494	2,239
Total Unrealized Losses	(42)	(21)
US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	31
Total Fair Value	31
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	2,276	1,784
Less than 12 months Unrealized Losses	(25)	(18)
12 months or more Fair Value	2
Total Fair Value	2,278	1,784
Total Unrealized Losses	(25)	(18)
Other Debt Securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	1,048	454
Less than 12 months Unrealized Losses	(13)	(3)
12 months or more Fair Value	100
12 months or more Unrealized Losses	(3)
Total Fair Value	1,148	454
Total Unrealized Losses	(16)	(3)
Other Debt And Equity Securities [Member]
Investments, Unrealized Loss Position
Less than 12 months Fair Value	37	1
Less than 12 months Unrealized Losses	(1)
Total Fair Value	37	1
Total Unrealized Losses	$ (1)