Loans and reserve for credit losses (Average Recorded Investment in Impaired Financing Receivables) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 12,946	$ 34,367	$ 49,980
Commercial Real Estate Owner Occupied
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	3,216	6,115	12,922
Commercial Real Estate Non Owner Occupied
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	6,384	22,699	25,655
Commercial real estate
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	9,600	28,814	38,577
Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	499	1,157	2,283
Residential real estate
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	54	381	1,596
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	2,793	4,015	7,039
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 0	$ 0	$ 485