Loans and reserve for credit losses (Troubled Debt Restructurings on Financing Receivables) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014 loan	Jun. 30, 2013 loan	Jun. 30, 2014 loan	Jun. 30, 2013 loan
Financing Receivable, Modifications [Line Items]
Number of loans	0	1	0	9
TDR outstanding recorded investment	$ 0	$ 3	$ 0	$ 27,954
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	0	0	0
TDR outstanding recorded investment	0	0	0	0
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	3	0	277
Consumer Loans
Financing Receivable, Modifications [Line Items]
Number of loans	0	0	0	0
TDR outstanding recorded investment	$ 0	$ 0	$ 0	$ 0