Loans and reserve for credit losses (Troubled Debt Restructurings on Financing Receivables) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014 loan	Sep. 30, 2013 loan	Sep. 30, 2014 loan	Sep. 30, 2013 loan
Financing Receivable, Modifications [Line Items]
Number of loans	0	2	0	10
TDR outstanding recorded investment	$ 0	$ 2,206	$ 0	$ 30,157
Commercial Real Estate
Financing Receivable, Modifications [Line Items]
Number of loans	0	0	0	5
TDR outstanding recorded investment	0	0	0	27,677
Construction
Financing Receivable, Modifications [Line Items]
Number of loans	0	1	0	1
TDR outstanding recorded investment	0	1,243	0	1,243
Residential real estate
Financing Receivable, Modifications [Line Items]
Number of loans	0	0	0	0
TDR outstanding recorded investment	0	0	0	0
Commercial and industrial loans
Financing Receivable, Modifications [Line Items]
Number of loans	0	1	0	4
TDR outstanding recorded investment	0	963	0	1,237
Consumer Loans
Financing Receivable, Modifications [Line Items]
Number of loans	0	0	0	0
TDR outstanding recorded investment	$ 0	$ 0	$ 0	$ 0