Loans and reserve for credit losses (Average Recorded Investment in Impaired Financing Receivables) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	$ 12,404	$ 19,117	$ 10,584	$ 23,984
Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	2,285	3,323	2,586	4,235
Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	2,554	12,121	1,918	15,709
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	4,839	15,444	4,504	19,944
Construction | Construction
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	632	122	742
Residential real estate | Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	99	6	171
Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	7,565	2,942	5,952	3,127
Consumer | Consumer
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	$ 0	$ 0	$ 0	$ 0