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Lost Interest (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2011
Financing Receivable, Impaired [Line Items]      
Amount by which interest income recognized on nonperforming loans exceeded reversals     $ 1
Amount by which interest income would have increased if nonperforming loans at period- end had been performing for the entire year $ 5 $ 4 $ 5