Loans and reserve for credit losses (Troubled Debt Restructurings on Financing Receivables) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) loan	Dec. 31, 2014 USD ($) loan	Dec. 31, 2013 USD ($) loan
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Number of loans | loan	1	0	10
TDR outstanding recorded investment | $	$ 25	$ 0	$ 30,157
Commercial real estate
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Number of loans | loan	0	0	5
TDR outstanding recorded investment | $	$ 0	$ 0	$ 27,677
Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Number of loans | loan	0	0	1
TDR outstanding recorded investment | $	$ 0	$ 0	$ 1,243
Residential real estate
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Number of loans | loan	0	0	0
TDR outstanding recorded investment | $	$ 0	$ 0	$ 0
Residential real estate
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Number of loans | loan	1	0	4
TDR outstanding recorded investment | $	$ 25	$ 0	$ 1,237
Commercial and industrial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Number of loans | loan	0	0	0
TDR outstanding recorded investment | $	$ 0	$ 0	$ 0