Investment Securities - Changes in Credit Losses on Debt Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Investments Debt And Equity Securities [Abstract]
Balance at beginning of period	$ 134	$ 298
Credit losses on securities not previously considered other-than-temporarily impaired		1
Decreases in expected cash flows on securities for which other-than-temporary impairment was previously recognized	7	8
Total other-than-temporary impairment on debt securities	7	9
Increases in expected cash flows		(6)
Realized losses	(8)	(13)
Balance at end of period	$ 133	$ 288