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
	Mar. 31, 2013	Mar. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Repurchase demands received from our legacy broker channel	$ 77	$ 84