Realized Gains and Losses Recognized in Income from Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Gain (loss) on investments
Realized gains	$ 77		$ 75		$ 75
Realized losses	(102)		(2)
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	74		58		19
Net realized gains	$ (99)	[1]	$ 15	[1]	$ 56	[1]

[1] Excludes net gains on interest-only mortgage-backed securities of $129 for the year ended December 31, 2013.