Loans and reserve for credit losses (Impaired Financing Receivables) (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances $ 4,225,000 $ 8,887,000
Impaired loans, Without a related allowance 29,497,000 31,580,000
Impaired loans, total recorded balance 33,722,000 40,467,000
Impaired loans, Unpaid principal balance 36,920,000 44,856,000
Impaired loans, Related allowance 85,000 783,000
Commercial Real Estate Owner Occupied    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 436,000 2,772,000
Impaired loans, Without a related allowance 5,624,000 6,582,000
Impaired loans, total recorded balance 6,060,000 9,354,000
Impaired loans, Unpaid principal balance 8,699,000 12,707,000
Impaired loans, Related allowance 41,000 652,000
Commercial Real Estate Non Owner Occupied    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 1,087,000 1,116,000
Impaired loans, Without a related allowance 21,800,000 21,757,000
Impaired loans, total recorded balance 22,887,000 22,873,000
Impaired loans, Unpaid principal balance 22,943,000 22,904,000
Impaired loans, Related allowance 19,000 13,000
Commercial real estate    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 1,523,000 3,888,000
Impaired loans, Without a related allowance 27,424,000 28,339,000
Impaired loans, total recorded balance 28,947,000 32,227,000
Impaired loans, Unpaid principal balance 31,642,000 35,611,000
Impaired loans, Related allowance 60,000 665,000
Construction    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 0 751,000
Impaired loans, Without a related allowance 963,000 1,236,000
Impaired loans, total recorded balance 963,000 1,987,000
Impaired loans, Unpaid principal balance 963,000 2,029,000
Impaired loans, Related allowance 0 0
Residential real estate    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 0 174,000
Impaired loans, Without a related allowance 317,000 256,000
Impaired loans, total recorded balance 317,000 430,000
Impaired loans, Unpaid principal balance 353,000 515,000
Impaired loans, Related allowance 0 62,000
Commercial and industrial    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 2,702,000 4,074,000
Impaired loans, Without a related allowance 793,000 1,749,000
Impaired loans, total recorded balance 3,495,000 5,823,000
Impaired loans, Unpaid principal balance 3,962,000 6,701,000
Impaired loans, Related allowance 25,000 56,000
Consumer    
Financing Receivable, Impaired [Line Items]    
Impaired loans with specific valuation allowances 0 0
Impaired loans, Without a related allowance 0 0
Impaired loans, total recorded balance 0 0
Impaired loans, Unpaid principal balance 0 0
Impaired loans, Related allowance $ 0 $ 0