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Acquisition Activity - Additional Information (Detail) - USD ($)
3 Months Ended
Aug. 23, 2017
Sep. 30, 2017
Mar. 31, 2018
Oct. 20, 2017
Dec. 31, 2016
Business Acquisition [Line Items]          
Share price       $ 42.00  
Total assets   $ 1,587,361,632     $ 1,556,732,033
Deposits   1,319,712,786     $ 1,248,072,453
Pre-tax merger-related expenses   $ 247,000      
St Martin Bancshares [Member]          
Business Acquisition [Line Items]          
Shares of common stock issued in merger 9.2839        
Cash distribution per share $ 94.00        
Stock consideration and special distribution, per share $ 483.92        
Stock consideration and special distribution, value $ 100,400,000        
Scenario, Forecast [Member]          
Business Acquisition [Line Items]          
Total assets     $ 2,200,000,000    
Loans     1,700,000,000    
Deposits     $ 1,800,000,000