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Securities - Summary of Contractual Maturities and Yields on Investments in Debt Securities (Detail) (Obligations of U.S. states and political subdivisions [Member], USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Obligations of U.S. states and political subdivisions [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Amortized cost due Within 1 Year   
Amortized cost After 1 but Within 5 Years   
Amortized cost After 5 but Within 10 Years   
Amortized cost, After 10 Years 1
Total amortized cost 1
Fair value Within 1 Year   
Fair value After 1 but Within 5 Years   
Fair value After 5 but Within 10 Years   
Fair value After 10 Years 1
Total fair value $ 1
Yield percentage due Within 1 Year   
Yield percentage After 1 but Within 5 Years   
Yield percentage After 5 but Within 10 Years   
Yield percentage After 10 Years 5.55%
Yield percentage, Total 5.55%