Guarantee Arrangements and Pledged Assets - Outstanding Repurchase Demands Received From GSEs and Other Third Parties (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Commitments And Guarantee Obligations [Line Items]
GSEs	$ 110	$ 77
Others	58	25
Total	$ 168	$ 102