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Acquisitions - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended
Oct. 17, 2017
Sep. 01, 2017
Feb. 03, 2017
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Aug. 31, 2017
Business Acquisition [Line Items]              
Common stock, shares outstanding       38,294,729 38,367,890    
Acquisition of goodwill       $ 119,880,000 $ 119,880,000    
Ownership interest percentage   5.00%          
Wolverine Bancorp Inc [Member]              
Business Acquisition [Line Items]              
Exchange ratio per share 152.28%            
Cash paid for each share $ 14.00            
Common stock, shares outstanding 2,129,331            
Common stock issued 3,241,045            
Market closing price per share $ 19.37            
Estimated transaction value $ 93,800,000            
Costs related to the acquisition 1,900,000            
Total estimated purchase price 93,773,000            
Net intangible assets acquired 2,024,000            
Acquisition of goodwill 26,827,000            
Purchase price of the business assets, portion deductible $ 0            
Core deposit intangible amortization period         10 years    
Lafayette Community Bancorp [Member]              
Business Acquisition [Line Items]              
Exchange ratio per share   88.17%          
Cash paid for each share   $ 1.73          
Common stock, shares outstanding   1,856,679          
Common stock issued   1,636,888          
Market closing price per share             $ 17.45
Estimated transaction value             $ 34,500,000
Costs related to the acquisition   $ 1,700,000          
Total estimated purchase price   34,465,000          
Net intangible assets acquired   2,085,000          
Acquisition of goodwill   15,408,000          
Purchase price of the business assets, portion deductible   $ 0          
Core deposit intangible amortization period         10 years    
Gain on remeasurement of equity interest       $ 530,000      
Payments received as cash consideration           $ (20,425,000)  
Lafayette Community Bancorp [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Number of shares owned   100          
Share of common stock outstanding per share   $ 17.25          
First Farmers Bank & Trust Co [Member]              
Business Acquisition [Line Items]              
Net intangible assets acquired     $ 452,000,000        
Acquisition of goodwill     $ 0        
Core deposit intangible amortization period     10 years        
Payments received as cash consideration     $ 11,000,000     $ (11,000,000)  
Business acquisition, loans assumed     $ 3,400,000        
Premium on deposits paid     3.00%        
Business acquisition, customer deposit balances     $ 14,800,000