Loans and reserve for credit losses (Schedule of Payment Defaults on Troubled Debt Restructurings on Financing Receivables) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($) loan
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction	$ 0	$ 3,983
Number of loans | loan		2
TDRs restructured in the period with a payment default		$ 3,500
Financing Receivables Troubled Debt Restructuring Effect Of Term Extension	25	4,674
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction and Term Extensions	0	21,500
Financing Receivables Modification Post Modification Recorded Investment	25	30,157
Commercial real estate
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction	0	$ 3,809
Number of loans | loan		2
TDRs restructured in the period with a payment default		$ 3,500
Financing Receivables Troubled Debt Restructuring Effect Of Term Extension	0	2,368
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction and Term Extensions	0	21,500
Financing Receivables Modification Post Modification Recorded Investment	0	27,677
Construction
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction	0	$ 0
Number of loans | loan		0
TDRs restructured in the period with a payment default		$ 0
Financing Receivables Troubled Debt Restructuring Effect Of Term Extension	0	1,243
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction and Term Extensions	0	0
Financing Receivables Modification Post Modification Recorded Investment	0	1,243
Residential real estate
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction	0	$ 0
Number of loans | loan		0
TDRs restructured in the period with a payment default		$ 0
Financing Receivables Troubled Debt Restructuring Effect Of Term Extension	0	0
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction and Term Extensions	0	0
Financing Receivables Modification Post Modification Recorded Investment	0	0
Residential real estate
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction	0	$ 174
Number of loans | loan		0
TDRs restructured in the period with a payment default		$ 0
Financing Receivables Troubled Debt Restructuring Effect Of Term Extension	25	1,063
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction and Term Extensions	0	0
Financing Receivables Modification Post Modification Recorded Investment	25	1,237
Commercial and industrial
Troubled Debt Restructuring, Debtor, Subsequent Periods [Line Items]
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction	0	$ 0
Number of loans | loan		0
TDRs restructured in the period with a payment default		$ 0
Financing Receivables Troubled Debt Restructuring Effect Of Term Extension	0	0
Financing Receivables, Troubled Debt Restructuring, Effect Of Rate Reduction and Term Extensions	0	0
Financing Receivables Modification Post Modification Recorded Investment	$ 0	$ 0