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Investment Securities - Additional Information (Detail)
9 Months Ended
Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]  
Federal statutory income tax rate 35.00%
Lower Limit
 
Schedule of Available-for-sale Securities [Line Items]  
Mortgage-backed securities contractual maturities (in years) 10
Upper Limit
 
Schedule of Available-for-sale Securities [Line Items]  
Mortgage-backed securities contractual maturities (in years) 30