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Commitments (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Commitments to extend credit    
Schedule of Commitments [Line Items]    
Off-balance sheet financial instruments, contractual amounts $ 1,411,819 $ 1,344,687
Letters of credit    
Schedule of Commitments [Line Items]    
Off-balance sheet financial instruments, contractual amounts $ 53,645 44,665
Letters of credit | Maximum    
Schedule of Commitments [Line Items]    
Remaining term 5 years  
Letters of credit | Minimum    
Schedule of Commitments [Line Items]    
Remaining term 1 month  
Unadvanced Commitment on Impaired Loan    
Schedule of Commitments [Line Items]    
Estimated losses attributable to unadvanced commitments on impaired loans $ 430 350
Fixed Rate Loan Commitment | Commitments to extend credit    
Schedule of Commitments [Line Items]    
Off-balance sheet financial instruments, contractual amounts $ 122,100 $ 68,500