Loans and reserve for credit losses (Average Recorded Investment in Impaired Financing Receivables) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	$ 7,533	$ 9,876
Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	565	2,876
Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	1,607
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	565	4,483
Construction | Construction
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	183
Residential real estate | Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	9
Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	6,968	5,201
Consumer | Consumer
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	$ 0	$ 0