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Loans and Loans Held for Sale - Additional Information (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
loan
Mar. 31, 2018
USD ($)
Sep. 30, 2017
USD ($)
loan
Sep. 30, 2018
USD ($)
loan
Sep. 30, 2017
USD ($)
loan
Dec. 31, 2017
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Loans, net of unearned income $ 5,400,000     $ 5,400,000   $ 5,200,000
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status       6 months    
Number of troubled debt restructuring loans returned to accruing status | loan 0   0 0 1  
Amount of trouble debt restructuring loans returned to accruing status $ 0   $ 0 $ 0 $ 2,000,000  
Number of commitments to lend additional funds on TDRs | loan 12     12    
Commitments to lend additional funds on TDRs $ 6,500,000     $ 6,500,000    
Minimum period of loan payment defaults following restructure for TDRs to be in default       90 days    
Number of defaulted TDRs that were restructured within the last twelve months prior to defaulting | loan 0   0 0 0  
Increase in other real estate       $ 2,600,000    
Land lots            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Increase in other real estate   $ 2,500,000        
Commercial            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Percentage of commercial loans in total portfolio loans 79.00%     79.00%   78.00%
Commercial loans $ 4,561,526,000     $ 4,561,526,000   $ 4,503,594,000
Commercial real estate and commercial construction | Commercial            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Commercial loans $ 3,100,000,000     $ 3,100,000,000   $ 3,100,000,000
Combined percentage of commercial real estate and commercial construction in total commercial loans 68.00%     68.00%   68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans 54.00%     54.00%   53.00%
Concentration risk percentage of commercial real estate and commercial construction       0.00%   0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of) 14.00%     14.00%   14.00%
Out of market exposure of combined portfolio (percent) 5.50%     5.50%   5.20%
Percentage of total loans out-of-state excluding contiguous states 2.90%     2.90%   2.80%