Guarantee Arrangements and Pledged Assets - Summary of Change in Estimated Repurchase Liability for Loans Sold to GSEs and Other Third Parties (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Balance at beginning of period	$ 219	$ 237
Increase in liability recorded through earnings	13	21
Realized losses	(21)	(35)
Balance at end of period	$ 211	$ 223