Loans and reserve for credit losses (Average Recorded Investment in Impaired Financing Receivables) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	$ 11,899	$ 7,318	$ 12,203	$ 14,946
Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	1,912	3,063	2,129	3,225
Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	2,525	1,072	2,540	8,297
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	4,437	4,135	4,669	11,522
Construction | Construction
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	443	0	543
Residential real estate | Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	0	28	0	66
Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	7,462	2,712	7,534	2,815
Consumer | Consumer
Financing Receivable, Recorded Investment [Line Items]
Impaired loans	$ 0	$ 0	$ 0	$ 0