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Credit Quality and Allowance for Loan Losses - Additional Information (Detail)
 9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
TDRs
Dec. 31, 2016
USD ($)
Financing Receivable, Impaired [Line Items]    
Financing receivable, recorded investment, 90 days past due and still accruing $ 0 $ 0
Maximum period of non-covered loans past due and accruing 90 days 90 days
Performing TDR not defaulted subsequent to restructuring or to the date financial statements available | TDRs 0  
Troubled debt restructuring performed $ 0  
GS Financial Corp [Member]    
Financing Receivable, Impaired [Line Items]    
Date of acquisition Jul. 15, 2011  
Britton & Koontz Capital Corporation [Member]    
Financing Receivable, Impaired [Line Items]    
Date of acquisition Feb. 14, 2014  
Louisiana Bancorp, Inc [Member]    
Financing Receivable, Impaired [Line Items]    
Date of acquisition Sep. 15, 2015