Loans and reserve for credit losses (Allowance for Credit Losses on Financing Receivables) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				$ 24,415					$ 24,415
Loan loss provision (credit)	$ 0	$ 0	$ 0	0	$ (2,000)	$ 0	$ 0	$ (2,000)	0	$ (4,000)	$ 0
Balance at end of period	25,290				24,415				25,290	24,415
Total reserve for credit losses	25,730				24,855				25,730	24,855	22,493
Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				24,415				22,053	24,415	22,053	20,857
Loan loss provision (credit)									0	(4,000)	0
Recoveries									6,003	9,874	6,439
Loans charged off									(5,128)	(3,512)	(5,243)
Balance at end of period	25,290				24,415				25,290	24,415	22,053
Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				440				440	440	440	440
Loan loss provision (credit)									0	0	0
Balance at end of period	440				440				440	440	440
Commercial real estate portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				3,934					3,934
Balance at end of period	6,321				3,934				6,321	3,934
Total reserve for credit losses	6,369				3,982				6,369	3,982	5,662
Commercial real estate portfolio | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				3,934				5,614	3,934	5,614	9,565
Loan loss provision (credit)									(530)	(5,552)	(4,484)
Recoveries									2,957	4,150	1,801
Loans charged off									(40)	(278)	(1,268)
Balance at end of period	6,321				3,934				6,321	3,934	5,614
Commercial real estate portfolio | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				48				48	48	48	48
Loan loss provision (credit)									0	0	0
Balance at end of period	48				48				48	48	48
Construction portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,044					1,044
Balance at end of period	2,196				1,044				2,196	1,044
Total reserve for credit losses	2,464				1,312				2,464	1,312	1,401
Construction portfolio | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,044				1,133	1,044	1,133	535
Loan loss provision (credit)									784	(525)	(348)
Recoveries									368	436	1,242
Loans charged off									0	0	(296)
Balance at end of period	2,196				1,044				2,196	1,044	1,133
Construction portfolio | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				268				268	268	268	268
Loan loss provision (credit)									0	0	0
Balance at end of period	268				268				268	268	268
Residential real estate portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				2,075					2,075
Balance at end of period	3,325				2,075				3,325	2,075
Total reserve for credit losses	3,350				2,100				3,350	2,100	2,146
Residential real estate portfolio | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				2,075				2,121	2,075	2,121	2,381
Loan loss provision (credit)									1,072	(494)	(315)
Recoveries									288	850	929
Loans charged off									(110)	(402)	(874)
Balance at end of period	3,325				2,075				3,325	2,075	2,121
Residential real estate portfolio | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				25				25	25	25	25
Loan loss provision (credit)									0	0	0
Balance at end of period	25				25				25	25	25
Commercial and industrial portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				13,969					13,969
Balance at end of period	10,657				13,969				10,657	13,969
Total reserve for credit losses	10,732				14,044				10,732	14,044	6,919
Commercial and industrial portfolio | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				13,969				6,844	13,969	6,844	6,261
Loan loss provision (credit)									(1,708)	4,682	(12)
Recoveries									1,470	3,820	2,158
Loans charged off									(3,074)	(1,377)	(1,563)
Balance at end of period	10,657				13,969				10,657	13,969	6,844
Commercial and industrial portfolio | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				75				75	75	75	75
Loan loss provision (credit)									0	0	0
Balance at end of period	75				75				75	75	75
Consumer portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				917					917
Balance at end of period	1,203				917				1,203	917
Total reserve for credit losses	1,227				941				1,227	941	1,071
Consumer portfolio | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				917				1,047	917	1,047	1,401
Loan loss provision (credit)									1,270	707	579
Recoveries									920	618	309
Loans charged off									(1,904)	(1,455)	(1,242)
Balance at end of period	1,203				917				1,203	917	1,047
Consumer portfolio | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				24				24	24	24	24
Loan loss provision (credit)									0	0	0
Balance at end of period	24				24				24	24	24
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				2,476					2,476
Balance at end of period	1,588				2,476				1,588	2,476
Total reserve for credit losses	1,588				2,476				1,588	2,476	5,294
Unallocated | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				2,476				5,294	2,476	5,294	714
Loan loss provision (credit)									(888)	(2,818)	4,580
Recoveries									0	0	0
Loans charged off									0	0	0
Balance at end of period	1,588				2,476				1,588	2,476	5,294
Unallocated | Unfunded Lending Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				$ 0				$ 0	0	0	0
Loan loss provision (credit)									0	0	0
Balance at end of period	$ 0				$ 0				$ 0	$ 0	$ 0