Loans and allowance for credit losses - Deferrals Program (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 464,832
Percentage of loans held for investment	6.40%
Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 25,641
Percentage of loans held for investment	1.80%
Construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 35,920
Percentage of loans held for investment	3.00%
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 43,281
Percentage of loans held for investment	3.80%
Residential real estate: | Residential line of credit
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 9,226
Percentage of loans held for investment	2.20%
Residential real estate: | Multi-family mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 19,005
Percentage of loans held for investment	11.50%
Commercial real estate: | Owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 60,614
Percentage of loans held for investment	6.60%
Commercial real estate: | Non-owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 270,552
Percentage of loans held for investment	16.50%
Consumer and other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Outstanding loans balance	$ 593
Percentage of loans held for investment	0.20%