Loans and reserve for credit losses (Impaired Financing Receivables) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	$ 5,442	$ 8,887
Impaired loans, Without a related allowance	28,046	31,580
Impaired loans, Total recorded balance	33,488	40,467
Impaired loans, Unpaid principal balance	36,634	44,856
Impaired loans, Related allowance	197	783
Commercial Real Estate Owner Occupied
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	1,614	2,772
Impaired loans, Without a related allowance	4,547	6,582
Impaired loans, Total recorded balance	6,161	9,354
Impaired loans, Unpaid principal balance	8,742	12,707
Impaired loans, Related allowance	153	652
Commercial Real Estate Non Owner Occupied
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	1,096	1,116
Impaired loans, Without a related allowance	21,290	21,757
Impaired loans, Total recorded balance	22,386	22,873
Impaired loans, Unpaid principal balance	22,439	22,904
Impaired loans, Related allowance	19	13
Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	2,710	3,888
Impaired loans, Without a related allowance	25,837	28,339
Impaired loans, Total recorded balance	28,547	32,227
Impaired loans, Unpaid principal balance	31,181	35,611
Impaired loans, Related allowance	172	665
Construction
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	0	751
Impaired loans, Without a related allowance	963	1,236
Impaired loans, Total recorded balance	963	1,987
Impaired loans, Unpaid principal balance	963	2,029
Impaired loans, Related allowance	0	0
Residential real estate
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	0	174
Impaired loans, Without a related allowance	322	256
Impaired loans, Total recorded balance	322	430
Impaired loans, Unpaid principal balance	357	515
Impaired loans, Related allowance	0	62
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Impaired loans, With specific valuation allowances	2,732	4,074
Impaired loans, Without a related allowance	924	1,749
Impaired loans, Total recorded balance	3,656	5,823
Impaired loans, Unpaid principal balance	4,133	6,701
Impaired loans, Related allowance	25	56
Consumer Loans
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