Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position
Less than 12 months Fair Value	$ 4,608	$ 2,239
Less than 12 months Unrealized Losses	(100)	(21)
12 months or more Fair Value	61
12 months or more Unrealized Losses	(2)
Total Fair Value	4,669	2,239
Total Unrealized Losses	(102)	(21)
US Treasury and Government
Investments, Unrealized Loss Position
Less than 12 months Fair Value	26
Total Fair Value	26
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position
Less than 12 months Fair Value	3,850	1,784
Less than 12 months Unrealized Losses	(88)	(18)
12 months or more Fair Value	3
Total Fair Value	3,853	1,784
Total Unrealized Losses	(88)	(18)
Other Debt Securities
Investments, Unrealized Loss Position
Less than 12 months Fair Value	702	454
Less than 12 months Unrealized Losses	(11)	(3)
12 months or more Fair Value	58
12 months or more Unrealized Losses	(2)
Total Fair Value	760	454
Total Unrealized Losses	(13)	(3)
Equity Securities, Other
Investments, Unrealized Loss Position
Less than 12 months Fair Value	30	1
Less than 12 months Unrealized Losses	(1)
Total Fair Value	30	1
Total Unrealized Losses	$ 1