Loans and reserve for credit losses (Impaired Financing Receivables) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	$ 8,571	$ 8,887
Impaired loans, Without a related allowance	27,953	31,580
Impaired loans,Total recorded balance	36,524	40,467
Impaired loans, Unpaid principal balance	39,662	44,856
Impaired loans, Related allowance	1,008	783
Commercial Real Estate Owner Occupied
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	2,754	2,772
Impaired loans, Without a related allowance	3,629	6,582
Impaired loans,Total recorded balance	6,383	9,354
Impaired loans, Unpaid principal balance	8,561	12,707
Impaired loans, Related allowance	647	652
Commercial Real Estate Non Owner Occupied
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	1,111	1,116
Impaired loans, Without a related allowance	21,627	21,757
Impaired loans,Total recorded balance	22,738	22,873
Impaired loans, Unpaid principal balance	22,771	22,904
Impaired loans, Related allowance	13	13
Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	3,865	3,888
Impaired loans, Without a related allowance	25,256	28,339
Impaired loans,Total recorded balance	29,121	32,227
Impaired loans, Unpaid principal balance	31,332	35,611
Impaired loans, Related allowance	660	665
Construction
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	689	751
Impaired loans, Without a related allowance	1,050	1,236
Impaired loans,Total recorded balance	1,739	1,987
Impaired loans, Unpaid principal balance	1,740	2,029
Impaired loans, Related allowance	0	0
Residential real estate
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	172	174
Impaired loans, Without a related allowance	254	256
Impaired loans,Total recorded balance	426	430
Impaired loans, Unpaid principal balance	513	515
Impaired loans, Related allowance	62	62
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Impaired loans with specific valuation allowances	3,845	4,074
Impaired loans, Without a related allowance	1,393	1,749
Impaired loans,Total recorded balance	5,238	5,823
Impaired loans, Unpaid principal balance	6,077	6,701
Impaired loans, Related allowance	286	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