Loans and reserve for credit losses (Allowance for Credit Losses on Financing Receivables) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			$ 22,053
Loan loss provision (credit)	$ 0	$ 0	(2,000)	$ 0
Allowance at the end of period	26,623		26,623
Total reserve for credit losses	27,063	21,791	27,063	21,791
Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	23,501	20,471	22,053	20,857
Loan loss provision (credit)	0	0	(2,000)	0
Recoveries	3,789	2,123	8,932	5,390
Loans charged off	(667)	(1,243)	(2,362)	(4,896)
Allowance at the end of period	26,623	21,351	26,623	21,351
Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	440	440	440	440
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	440	440	440	440
Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			5,614
Allowance at the end of period	4,950		4,950
Total reserve for credit losses	4,998	7,031	4,998	7,031
Commercial Real Estate | Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	5,032	7,960	5,614	9,565
Loan loss provision (credit)	(490)	(1,203)	(4,397)	(2,681)
Recoveries	408	317	4,011	1,318
Loans charged off	0	(91)	(278)	(1,219)
Allowance at the end of period	4,950	6,983	4,950	6,983
Commercial Real Estate | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	48	48	48	48
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	48	48	48	48
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			1,133
Allowance at the end of period	1,218		1,218
Total reserve for credit losses	1,486	1,268	1,486	1,268
Construction | Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	1,356	728	1,133	535
Loan loss provision (credit)	(293)	(597)	(193)	(482)
Recoveries	155	869	278	1,243
Loans charged off	0	0	0	(296)
Allowance at the end of period	1,218	1,000	1,218	1,000
Construction | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	268	268	268	268
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	268	268	268	268
Residential real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			2,121
Allowance at the end of period	2,413		2,413
Total reserve for credit losses	2,438	1,909	2,438	1,909
Residential real estate | Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	2,463	2,068	2,121	2,381
Loan loss provision (credit)	(162)	6	(60)	(264)
Recoveries	162	349	746	630
Loans charged off	(50)	(539)	(394)	(863)
Allowance at the end of period	2,413	1,884	2,413	1,884
Residential real estate | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	25	25	25	25
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	25	25	25	25
Commercial and Industrial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			6,844
Allowance at the end of period	11,702		11,702
Total reserve for credit losses	11,777	7,427	11,777	7,427
Commercial and Industrial | Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	11,355	6,615	6,844	6,261
Loan loss provision (credit)	(2,245)	469	2,024	706
Recoveries	2,885	512	3,440	1,969
Loans charged off	(293)	(244)	(606)	(1,584)
Allowance at the end of period	11,702	7,352	11,702	7,352
Commercial and Industrial | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	75	75	75	75
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	75	75	75	75
Consumer Loans
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			1,047
Allowance at the end of period	955		955
Total reserve for credit losses	979	1,123	979	1,123
Consumer Loans | Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	1,001	1,181	1,047	1,401
Loan loss provision (credit)	99	211	535	402
Recoveries	179	76	457	230
Loans charged off	(324)	(369)	(1,084)	(934)
Allowance at the end of period	955	1,099	955	1,099
Consumer Loans | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	24	24	24	24
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	24	24	24	24
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			5,294
Allowance at the end of period	5,385		5,385
Total reserve for credit losses	5,385	3,033	5,385	3,033
Unallocated | Loan Receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	2,294	1,919	5,294	714
Loan loss provision (credit)	3,091	1,114	$ 91	2,319
Recoveries	0			0
Loans charged off	0			0
Allowance at the end of period	5,385	3,033	$ 5,385	3,033
Unallocated | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	0	0	0	0
Loan loss provision (credit)	0	0	0	0
Allowance at the end of period	$ 0	$ 0	$ 0	$ 0