Loans - High LTV Interest-only and Arm Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified					3 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Sub-prime residential mortgage [Member]	Dec. 31, 2011 Sub-prime residential mortgage [Member]	Dec. 31, 2012 Subsequent	Dec. 31, 2013 Subsequent
Loans [Line Items]
Residential mortgage loans with high LTV and no mortgage insurance	$ 900	$ 1,100	$ 34	$ 68
Sub-prime residential mortgage loans held for sale excluded from Arm Loan			23	28
Amount of ARM loans to experience first interest rate reset					$ 43	$ 334