Loans and allowance for loan losses- Loans Outstanding by Major Lending Classification (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	$ 3,667,511	$ 3,166,911
Less: Allowance for loan losses	(28,932)	(24,041)	$ (21,747)	$ (24,460)
Net loans	3,638,579	3,142,870
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	867,083	715,075
Less: Allowance for loan losses	(5,348)	(4,461)	(5,309)	(5,135)
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	556,051	448,326
Less: Allowance for loan losses	(9,729)	(7,135)	(4,940)	(5,143)
Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	555,815	480,989
Less: Allowance for loan losses	(3,428)	(3,197)	(3,197)	(4,176)
Residential real estate: \| Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	190,480	194,986
Less: Allowance for loan losses	(811)	(944)	(1,613)	(2,201)
Residential real estate: \| Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	75,457	62,374
Less: Allowance for loan losses	(566)	(434)	(504)	(311)
Commercial real estate: \| Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	493,524	495,872
Less: Allowance for loan losses	(3,132)	(3,558)	(3,302)	(3,682)
Commercial real estate: \| Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	700,248	551,588
Less: Allowance for loan losses	(4,149)	(2,817)	(2,019)	(2,622)
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	228,853	217,701
Less: Allowance for loan losses	$ (1,769)	$ (1,495)	$ (863)	$ (1,190)

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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 before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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