Loans, Allowance for Credit Losses - Loans, and Credit Quality - Nonaccrual loans by portfolio segment (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 14,769	$ 14,122
Percent of total	100.00%	100.00%
Percent of total loans	0.60%	0.50%
Retail & other
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 1
Percent of total	0.00%	0.00%
Commercial portfolio segment | Commercial & industrial
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 6,050	$ 6,249
Percent of total	41.00%	44.00%
Commercial portfolio segment | Owner-occupied CRE
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 3,837	$ 3,311
Percent of total	26.00%	23.00%
Commercial portfolio segment | Agricultural
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 1,801	$ 1,898
Percent of total	12.00%	14.00%
Commercial real estate portfolio segment | CRE investment
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 1,029	$ 1,073
Percent of total	7.00%	8.00%
Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 533	$ 20
Percent of total	4.00%	0.00%
Residential | Residential first mortgage
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 953	$ 1,090
Percent of total	6.00%	8.00%
Residential | Residential junior mortgage
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 566	$ 480
Percent of total	4.00%	3.00%
Residential | Residential construction
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
Percent of total	0.00%	0.00%