Loans and allowance for loan losses - Allowance for Loan Losses by Portfolio Segment and Related Investment in Loans Net of Unearned Interest (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 24,041	$ 21,747	$ 24,460
Provision for loan losses	5,398	(950)	(1,479)
Recoveries of loans previously charged-off	2,647	5,771	1,616
Loans charged off	(2,805)	(2,527)	(2,850)
Adjustments for transfers to loans HFS	(349)
Ending balance	28,932	24,041	21,747
Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,461	5,309	5,135
Provision for loan losses	1,395	(2,158)	212
Recoveries of loans previously charged-off	390	1,894	524
Loans charged off	(898)	(584)	(562)
Adjustments for transfers to loans HFS	0
Ending balance	5,348	4,461	5,309
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,135	4,940	5,143
Provision for loan losses	1,459	1,138	(417)
Recoveries of loans previously charged-off	1,164	1,084	216
Loans charged off	(29)	(27)	(2)
Adjustments for transfers to loans HFS	0
Ending balance	9,729	7,135	4,940
Residential real estate: \| 1-to-4 family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,197	3,197	4,176
Provision for loan losses	547	41	(882)
Recoveries of loans previously charged-off	171	159	127
Loans charged off	(138)	(200)	(224)
Adjustments for transfers to loans HFS	(349)
Ending balance	3,428	3,197	3,197
Residential real estate: \| Residential line of credit
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	944	1,613	2,201
Provision for loan losses	(275)	(788)	(630)
Recoveries of loans previously charged-off	178	395	174
Loans charged off	(36)	(276)	(132)
Adjustments for transfers to loans HFS	0
Ending balance	811	944	1,613
Residential real estate: \| Multi-family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	434	504	311
Provision for loan losses	132	(70)	193
Recoveries of loans previously charged-off	0	0	0
Loans charged off	0	0	0
Adjustments for transfers to loans HFS	0
Ending balance	566	434	504
Commercial real estate: \| Owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,558	3,302	3,682
Provision for loan losses	(478)	483	(271)
Recoveries of loans previously charged-off	143	61	140
Loans charged off	(91)	(288)	(249)
Adjustments for transfers to loans HFS	0
Ending balance	3,132	3,558	3,302
Commercial real estate: \| Non-owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,817	2,019	2,622
Provision for loan losses	1,281	(848)	(271)
Recoveries of loans previously charged-off	51	1,646	195
Loans charged off	0	0	(527)
Adjustments for transfers to loans HFS	0
Ending balance	4,149	2,817	2,019
Consumer and other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,495	863	1,190
Provision for loan losses	1,337	1,252	587
Recoveries of loans previously charged-off	550	532	240
Loans charged off	(1,613)	(1,152)	(1,154)
Adjustments for transfers to loans HFS	0
Ending balance	$ 1,769	$ 1,495	$ 863

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

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Amount of other addition (reduction) in allowance reserve for credit loss.

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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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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.

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

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