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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			$ 24,415
Loan loss recovery	$ 0	$ 0	0	$ (2,000)
Allowance at the end of period	25,238		25,238
Total reserve for credit losses	25,678	27,063	25,678	27,063
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			2,476
Allowance at the end of period	1,838		1,838
Total reserve for credit losses	1,838	5,385	1,838	5,385
Commercial real estate: \| Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			3,934
Allowance at the end of period	6,019		6,019
Total reserve for credit losses	6,067	4,998	6,067	4,998
Construction \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			1,044
Allowance at the end of period	1,857		1,857
Total reserve for credit losses	2,125	1,486	2,125	1,486
Residential real estate \| Residential real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			2,075
Allowance at the end of period	3,763		3,763
Total reserve for credit losses	3,788	2,438	3,788	2,438
Commercial and industrial \| Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			13,969
Allowance at the end of period	10,549		10,549
Total reserve for credit losses	10,624	11,777	10,624	11,777
Consumer \| Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period			917
Allowance at the end of period	1,212		1,212
Total reserve for credit losses	1,236	979	1,236	979
Loan receivables
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	24,666	23,501	24,415	22,053
Loan loss recovery	0	0	0	(2,000)
Recoveries	1,082	3,789	5,325	8,932
Loans charged off	(510)	(667)	(4,502)	(2,362)
Allowance at the end of period	25,238	26,623	25,238	26,623
Loan receivables \| Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	3,382	2,294	2,476	5,294
Loan loss recovery	(1,544)	3,091	(638)	91
Recoveries	0	0	0	0
Loans charged off	0	0	0	0
Allowance at the end of period	1,838	5,385	1,838	5,385
Loan receivables \| Commercial real estate: \| Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	4,391	5,032	3,934	5,614
Loan loss recovery	1,554	(490)	(667)	(4,397)
Recoveries	74	408	2,792	4,011
Loans charged off	0	0	(40)	(278)
Allowance at the end of period	6,019	4,950	6,019	4,950
Loan receivables \| Construction \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	1,259	1,356	1,044	1,133
Loan loss recovery	271	(293)	407	(193)
Recoveries	327	155	406	278
Loans charged off	0	0	0	0
Allowance at the end of period	1,857	1,218	1,857	1,218
Loan receivables \| Residential real estate \| Residential real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	2,674	2,463	2,075	2,121
Loan loss recovery	1,044	(162)	1,530	(60)
Recoveries	45	162	222	746
Loans charged off	0	(50)	(64)	(394)
Allowance at the end of period	3,763	2,413	3,763	2,413
Loan receivables \| Commercial and industrial \| Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	11,982	11,355	13,969	6,844
Loan loss recovery	(1,766)	(2,245)	(1,554)	2,024
Recoveries	428	2,885	1,190	3,440
Loans charged off	(95)	(293)	(3,056)	(606)
Allowance at the end of period	10,549	11,702	10,549	11,702
Loan receivables \| Consumer \| Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	978	1,001	917	1,047
Loan loss recovery	441	99	922	535
Recoveries	208	179	715	457
Loans charged off	(415)	(324)	(1,342)	(1,084)
Allowance at the end of period	1,212	955	1,212	955
Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	440	440	440	440
Loan loss recovery	0	0	0	0
Allowance at the end of period	440	440	440	440
Reserve for unfunded lending commitments \| Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	0	0	0	0
Loan loss recovery	0	0	0	0
Allowance at the end of period	0	0	0	0
Reserve for unfunded lending commitments \| Commercial real estate: \| Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	48	48	48	48
Loan loss recovery	0	0	0	0
Allowance at the end of period	48	48	48	48
Reserve for unfunded lending commitments \| Construction \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	268	268	268	268
Loan loss recovery	0	0	0	0
Allowance at the end of period	268	268	268	268
Reserve for unfunded lending commitments \| Residential real estate \| Residential real estate
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	25	25	25	25
Loan loss recovery	0	0	0	0
Allowance at the end of period	25	25	25	25
Reserve for unfunded lending commitments \| Commercial and industrial \| Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	75	75	75	75
Loan loss recovery	0	0	0	0
Allowance at the end of period	75	75	75	75
Reserve for unfunded lending commitments \| Consumer \| Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance at the beginning of period	24	24	24	24
Loan loss recovery	0	0	0	0
Allowance at the end of period	$ 24	$ 24	$ 24	$ 24

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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A valuation allowance for financing receivables that are expected to be uncollectible.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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- Definition

Amount of expense related to estimated loss from loan and lease transactions.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
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