Loans and allowance for loan losses- Loans Outstanding by Major Lending Classification (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	$ 3,786,791	$ 3,667,511
Less: Allowance for loan losses	(29,814)	(28,932)	$ (24,406)	$ (24,041)
Net loans	3,756,977	3,638,579
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	888,345	867,083
Less: Allowance for loan losses	(5,514)	(5,348)	(4,578)	(4,461)
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	539,065	556,051
Less: Allowance for loan losses	(9,758)	(9,729)	(7,866)	(7,135)
Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	552,239	555,815
Less: Allowance for loan losses	(3,295)	(3,428)	(3,122)	(3,197)
Residential real estate: \| Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	187,415	190,480
Less: Allowance for loan losses	(731)	(811)	(1,165)	(944)
Residential real estate: \| Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	71,532	75,457
Less: Allowance for loan losses	(539)	(566)	(449)	(434)
Commercial real estate: \| Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	499,123	493,524
Less: Allowance for loan losses	(3,098)	(3,132)	(3,014)	(3,558)
Commercial real estate: \| Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	816,880	700,248
Less: Allowance for loan losses	(4,583)	(4,149)	(2,753)	(2,817)
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Gross loans	232,192	228,853
Less: Allowance for loan losses	$ (2,296)	$ (1,769)	$ (1,459)	$ (1,495)

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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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