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Allowance for Loan Losses - Additional Information (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Jul. 01, 2006
Receivables [Abstract]      
Forbearance period after grace period 6 months    
Period after grace period for forbearance allowance 9 months    
Period of forbearance within 24-month period to be classified as TDR 3 months    
Percentage of loans granted forbearance qualified as TDR 22.00% 10.00%  
Troubled debt restructuring $ 251.0    
Criteria for loans to be considered as nonperforming 90 days    
Tier 1 of government guarantee 97.00%   97.00%
TDR payment default period 60 days    
Ffelp loans $ 700.0    
Percentage of FFELP rehabilitation loans 61.00% 62.00%  
Period of loans past due that have accrued interest 90 days