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Fair Values of Assets and Liabilities - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Standby Letters of Credit and Unfunded Commitments [Member]
Dec. 31, 2012
Standby Letters of Credit and Unfunded Commitments [Member]
Sep. 30, 2013
Other Guarantees [Member]
Dec. 31, 2012
Other Guarantees [Member]
Sep. 30, 2013
Minimum [Member]
Sep. 30, 2013
Maximum [Member]
Sep. 30, 2013
Average [Member]
Fair Value Disclosures [Abstract]                      
Fair value measurement transfers from one level to another     $ 0 $ 0              
Mortgage loans held for sale measured at fair value gain (loss) (5) 241 (326) 501              
Assumed credit valuation adjustment as percentage of underlying fair value of underlying derivative contracts                 0.00% 95.00% 6.00%
Carrying value of guarantee obligations         $ 410 $ 415 $ 368 $ 452