Loans, Allowance for Credit Losses - Loans, and Credit Quality - Collateral dependent loans by portfolio segment (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 8,075
Without an Allowance	4,143
With an Allowance	3,932
Allowance Allocation	1,012
Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	4,041
Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	4,034
Retail & other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Without an Allowance	0
With an Allowance	0
Allowance Allocation	0
Retail & other | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Retail & other | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Commercial portfolio segment | Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	2,579
Without an Allowance	0
With an Allowance	2,579
Allowance Allocation	848
Commercial portfolio segment | Commercial & industrial | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Commercial portfolio segment | Commercial & industrial | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	2,579
Commercial portfolio segment | PPP loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Without an Allowance	0
With an Allowance	0
Allowance Allocation	0
Commercial portfolio segment | PPP loans | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Commercial portfolio segment | PPP loans | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Commercial portfolio segment | Owner-occupied commercial real estate (“CRE”)
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,986
Without an Allowance	1,986
With an Allowance	0
Allowance Allocation	0
Commercial portfolio segment | Owner-occupied commercial real estate (“CRE”) | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,986
Commercial portfolio segment | Owner-occupied commercial real estate (“CRE”) | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Commercial portfolio segment | Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	2,080
Without an Allowance	727
With an Allowance	1,353
Allowance Allocation	164
Commercial portfolio segment | Agricultural | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	625
Commercial portfolio segment | Agricultural | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,455
Commercial real estate portfolio segment | CRE investment
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	897
Without an Allowance	897
With an Allowance	0
Allowance Allocation	0
Commercial real estate portfolio segment | CRE investment | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	897
Commercial real estate portfolio segment | CRE investment | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	533
Without an Allowance	533
With an Allowance	0
Allowance Allocation	0
Commercial real estate portfolio segment | Construction & land development | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	533
Commercial real estate portfolio segment | Construction & land development | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Residential | Residential construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Without an Allowance	0
With an Allowance	0
Allowance Allocation	0
Residential | Residential construction | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Residential | Residential construction | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Residential | Residential first mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Without an Allowance	0
With an Allowance	0
Allowance Allocation	0
Residential | Residential first mortgage | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Residential | Residential first mortgage | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Residential | Residential junior mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Without an Allowance	0
With an Allowance	0
Allowance Allocation	0
Residential | Residential junior mortgage | Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Residential | Residential junior mortgage | Other Business Assets
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 0