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Allowance for Loan and Lease Losses and Reserve for Unfunded Commitments - Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Loans And Leases Receivable Disclosure [Line Items]      
Increase (decrease) in allowance for loan and lease losses $ 5,800,000 $ 3,300,000  
Commitments outstanding on troubled debt restructurings 0 0  
Recorded investment in troubled debt restructurings that subsequently defaulted within twelve months 144,000 477,000 $ 750,000
Troubled debt restructurings classified as non-accrual 1,600,000 552,000  
Reserve for unfunded commitments 923,000 760,000  
Provisions for unfunded commitments 162,000 493,000  
Charge-offs or recoveries related to reserve for unfunded commitments 0 0  
Acquired Impaired Loans      
Loans And Leases Receivable Disclosure [Line Items]      
Increase (decrease) in allowance for loan and lease losses $ 2,300,000 $ (1,700,000)