Guarantee Arrangements and Pledged Assets - Trend in Repurchase Demands Received on Loans Sold to GSEs and Other Third Parties by Loan Origination Vintage (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Commitments And Guarantee Obligations [Line Items]
Repurchase demands received from our legacy broker channel	$ 98	$ 61	$ 330	$ 218