Loans and allowance for loan losses- Loans Outstanding by Major Lending Classification (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Past Due [Line Items]
Gross loans	$ 4,409,642	$ 3,667,511
Less: Allowance for loan losses	(31,139)	(28,932)	$ (24,041)	$ (21,747)
Net loans	4,378,503	3,638,579
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Gross loans	1,034,036	867,083
Less: Allowance for loan losses	(4,805)	(5,348)	(4,461)	(5,309)
Construction
Financing Receivable, Past Due [Line Items]
Gross loans	551,101	556,051
Less: Allowance for loan losses	(10,194)	(9,729)	(7,135)	(4,940)
Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	710,454	555,815
Less: Allowance for loan losses	(3,112)	(3,428)	(3,197)	(3,197)
Residential real estate: \| Residential line of credit
Financing Receivable, Past Due [Line Items]
Gross loans	221,530	190,480
Less: Allowance for loan losses	(752)	(811)	(944)	(1,613)
Residential real estate: \| Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	69,429	75,457
Less: Allowance for loan losses	(544)	(566)	(434)	(504)
Commercial real estate: \| Owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	630,270	493,524
Less: Allowance for loan losses	(4,109)	(3,132)	(3,558)	(3,302)
Commercial real estate: \| Non-owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	920,744	700,248
Less: Allowance for loan losses	(4,621)	(4,149)	(2,817)	(2,019)
Consumer and other
Financing Receivable, Past Due [Line Items]
Gross loans	272,078	228,853
Less: Allowance for loan losses	$ (3,002)	$ (1,769)	$ (1,495)	$ (863)

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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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