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	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 29,814	$ 24,406	$ 28,932	$ 24,041
Provision for loan losses	881	1,063	2,272	1,380
Recoveries of loans previously charged-off	213	1,299	575	2,008
Loans charged off	(770)	(421)	(1,641)	(1,082)
Ending balance	30,138	26,347	30,138	26,347
Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,514	4,578	5,348	4,461
Provision for loan losses	(550)	39	(217)	241
Recoveries of loans previously charged-off	38	135	50	270
Loans charged off	(79)	(5)	(258)	(225)
Ending balance	4,923	4,747	4,923	4,747
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,758	7,866	9,729	7,135
Provision for loan losses	(109)	310	(81)	789
Recoveries of loans previously charged-off	6	862	7	1,114
Loans charged off	0	(15)	0	(15)
Ending balance	9,655	9,023	9,655	9,023
Residential real estate: \| 1-to-4 family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,295	3,122	3,428	3,197
Provision for loan losses	(30)	218	(95)	188
Recoveries of loans previously charged-off	24	43	37	58
Loans charged off	(1)	(5)	(82)	(65)
Ending balance	3,288	3,378	3,288	3,378
Residential real estate: \| Residential line of credit
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	731	1,165	811	944
Provision for loan losses	106	(414)	33	(200)
Recoveries of loans previously charged-off	21	44	46	71
Loans charged off	(103)	0	(135)	(20)
Ending balance	755	795	755	795
Residential real estate: \| Multi-family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	539	449	566	434
Provision for loan losses	78	(58)	51	(43)
Recoveries of loans previously charged-off	0	0	0	0
Loans charged off	0	0	0	0
Ending balance	617	391	617	391
Commercial real estate: \| Owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,098	3,014	3,132	3,558
Provision for loan losses	409	168	288	(399)
Recoveries of loans previously charged-off	5	108	92	131
Loans charged off	0	0	0	0
Ending balance	3,512	3,290	3,512	3,290
Commercial real estate: \| Non-owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,583	2,753	4,149	2,817
Provision for loan losses	(105)	519	329	404
Recoveries of loans previously charged-off	0	0	0	51
Loans charged off	0	0	0	0
Ending balance	4,478	3,272	4,478	3,272
Consumer and other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,296	1,459	1,769	1,495
Provision for loan losses	1,082	281	1,964	400
Recoveries of loans previously charged-off	119	107	343	313
Loans charged off	(587)	(396)	(1,166)	(757)
Ending balance	$ 2,910	$ 1,451	$ 2,910	$ 1,451

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.

+ References

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

+ References

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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

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

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