Loans, Allowance for Credit Losses - Loans, and Credit Quality - Summary of loans by past due status (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 2,895,162	$ 2,552,360
Total	$ 2,908,793	$ 2,573,751
Percent of current loans	99.50%	99.20%
Percent of total loans	100.00%	100.00%
30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	$ 2,634	$ 7,269
Percent past due	0.10%	0.30%
90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	$ 10,997	$ 14,122
Percent past due	0.40%	0.50%
Retail & other
Financing Receivable, Past Due [Line Items]
Current	$ 32,564	$ 30,100
Total	33,016	30,211
Retail & other | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	401	110
Retail & other | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	51	1
Commercial portfolio segment | Commercial & industrial
Financing Receivable, Past Due [Line Items]
Current	732,480	798,211
Total	735,531	806,189
Commercial portfolio segment | Commercial & industrial | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	40	1,729
Commercial portfolio segment | Commercial & industrial | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	3,011	6,249
Commercial portfolio segment | PPP loans
Financing Receivable, Past Due [Line Items]
Current	335,236
Total	335,236	0
Commercial portfolio segment | PPP loans | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	0
Commercial portfolio segment | PPP loans | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	0
Commercial portfolio segment | Owner-occupied CRE
Financing Receivable, Past Due [Line Items]
Current	495,754	492,949
Total	499,605	496,372
Commercial portfolio segment | Owner-occupied CRE | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	1,380	112
Commercial portfolio segment | Owner-occupied CRE | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	2,471	3,311
Commercial portfolio segment | Agricultural
Financing Receivable, Past Due [Line Items]
Current	108,598	93,552
Total	111,022	95,450
Commercial portfolio segment | Agricultural | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	127	0
Commercial portfolio segment | Agricultural | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	2,297	1,898
Commercial real estate portfolio segment | CRE investment
Financing Receivable, Past Due [Line Items]
Current	474,139	442,145
Total	475,050	443,218
Commercial real estate portfolio segment | CRE investment | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	0	0
Commercial real estate portfolio segment | CRE investment | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	911	1,073
Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Past Due [Line Items]
Current	121,046	90,887
Total	121,647	92,970
Commercial real estate portfolio segment | Construction & land development | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	68	2,063
Commercial real estate portfolio segment | Construction & land development | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	533	20
Residential | Residential construction
Financing Receivable, Past Due [Line Items]
Current	57,496	54,101
Total	57,496	54,403
Residential | Residential construction | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	0	302
Residential | Residential construction | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	0	0
Residential | Residential first mortgage
Financing Receivable, Past Due [Line Items]
Current	426,162	428,341
Total	428,017	432,167
Residential | Residential first mortgage | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	543	2,736
Residential | Residential first mortgage | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	1,312	1,090
Residential | Residential junior mortgage
Financing Receivable, Past Due [Line Items]
Current	111,687	122,074
Total	112,173	122,771
Residential | Residential junior mortgage | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	75	217
Residential | Residential junior mortgage | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	$ 411	$ 480