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Commitments, Credit Risk and Contingencies - Summary of Off-Balance Sheet Credit Risk Information (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Commitments to Extend Credit [Member]
   
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Extended credit and standby letters of credit $ 2,822,545 $ 2,600,355
Standby Letters of Credit [Member]
   
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Extended credit and standby letters of credit $ 122,713 $ 130,912