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Commitments (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Commitments to extend credit    
Schedule of Commitments [Line Items]    
Off-balance sheet financial instruments, contractual amounts $ 1,298,423 $ 1,075,170
Letters of credit    
Schedule of Commitments [Line Items]    
Off-balance sheet financial instruments, contractual amounts $ 73,790 78,954
Letters of credit | Minimum    
Schedule of Commitments [Line Items]    
Remaining term 1 month  
Letters of credit | Maximum    
Schedule of Commitments [Line Items]    
Remaining term 3 years 9 months  
Unadvanced Commitment on Impaired Loan    
Schedule of Commitments [Line Items]    
Estimated losses attributable to unadvanced commitments on impaired loans $ 400  
Fixed Rate Loan Commitment | Commitments to extend credit    
Schedule of Commitments [Line Items]    
Off-balance sheet financial instruments, contractual amounts $ 112,000 $ 89,700