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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				$ 22,053					$ 22,053
Loan loss provision (credit)	$ (2,000)	$ 0	$ 0	(2,000)	$ 0	$ 0	$ 0	$ 0	(4,000)	$ 0	$ 1,000
Balance at end of period	24,415				22,053				24,415	22,053
Total reserve for credit losses	24,855				22,493				24,855	22,493	21,297
Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				22,053				20,857	22,053	20,857	27,261
Loan loss provision (credit)									(4,000)	0	1,000
Recoveries									9,874	6,439	5,132
Loans charged off									(3,512)	(5,243)	(12,536)
Balance at end of period	24,415				22,053				24,415	22,053	20,857
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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				5,614					5,614
Balance at end of period	3,934				5,614				3,934	5,614
Total reserve for credit losses	3,982				5,662				3,982	5,662	9,613
Commercial real estate | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				5,614				9,565	5,614	9,565	11,596
Loan loss provision (credit)									(5,552)	(4,484)	203
Recoveries									4,150	1,801	1,034
Loans charged off									(278)	(1,268)	(3,268)
Balance at end of period	3,934				5,614				3,934	5,614	9,565
Commercial real estate | 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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,133					1,133
Balance at end of period	1,044				1,133				1,044	1,133
Total reserve for credit losses	1,312				1,401				1,312	1,401	803
Construction | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,133				535	1,133	535	1,583
Loan loss provision (credit)									(525)	(348)	159
Recoveries									436	1,242	708
Loans charged off									0	(296)	(1,915)
Balance at end of period	1,044				1,133				1,044	1,133	535
Construction | 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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				2,121					2,121
Balance at end of period	2,075				2,121				2,075	2,121
Total reserve for credit losses	2,100				2,146				2,100	2,146	2,406
Residential real estate | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				2,121				2,381	2,121	2,381	3,551
Loan loss provision (credit)									(494)	(315)	(1,013)
Recoveries									850	929	433
Loans charged off									(402)	(874)	(590)
Balance at end of period	2,075				2,121				2,075	2,121	2,381
Residential real estate | 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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				6,844					6,844
Balance at end of period	13,969				6,844				13,969	6,844
Total reserve for credit losses	14,044				6,919				14,044	6,919	6,336
Commercial and industrial | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				6,844				6,261	6,844	6,261	7,267
Loan loss provision (credit)									4,682	(12)	1,808
Recoveries									3,820	2,158	2,694
Loans charged off									(1,377)	(1,563)	(5,508)
Balance at end of period	13,969				6,844				13,969	6,844	6,261
Commercial and industrial | 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
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,047					1,047
Balance at end of period	917				1,047				917	1,047
Total reserve for credit losses	941				1,071				941	1,071	1,425
Consumer | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,047				1,401	1,047	1,401	2,177
Loan loss provision (credit)									707	579	216
Recoveries									618	309	263
Loans charged off									(1,455)	(1,242)	(1,255)
Balance at end of period	917				1,047				917	1,047	1,401
Consumer | 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				5,294					5,294
Balance at end of period	2,476				5,294				2,476	5,294
Total reserve for credit losses	2,476				5,294				2,476	5,294	714
Unallocated | Loan Losses
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				5,294				714	5,294	714	1,087
Loan loss provision (credit)									(2,818)	4,580	(373)
Recoveries									0	0	0
Loans charged off									0	0	0
Balance at end of period	2,476				5,294				2,476	5,294	714
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