Loans, Allowance for Credit Losses - Loans, and Credit Quality - Summary of loans by past due status (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 2,808,051	$ 2,552,360
Total	$ 2,821,501	$ 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	$ 1,452	$ 7,269
Percent past due	0.10%	0.30%
90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	$ 11,998	$ 14,122
Percent past due	0.40%	0.50%
Retail & other
Financing Receivable, Past Due [Line Items]
Current	$ 29,727	$ 30,100
Total	29,812	30,211
Retail & other | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	85	110
Retail & other | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	0	1
Commercial portfolio segment | Commercial & industrial
Financing Receivable, Past Due [Line Items]
Current	724,960	798,211
Total	729,264	806,189
Commercial portfolio segment | Commercial & industrial | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	162	1,729
Commercial portfolio segment | Commercial & industrial | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	4,142	6,249
Commercial portfolio segment | PPP loans
Financing Receivable, Past Due [Line Items]
Current	329,157
Total	329,157	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	492,646	492,949
Total	495,722	496,372
Commercial portfolio segment | Owner-occupied CRE | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	71	112
Commercial portfolio segment | Owner-occupied CRE | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	3,005	3,311
Commercial portfolio segment | Agricultural
Financing Receivable, Past Due [Line Items]
Current	97,274	93,552
Total	99,020	95,450
Commercial portfolio segment | Agricultural | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	35	0
Commercial portfolio segment | Agricultural | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	1,711	1,898
Commercial real estate portfolio segment | CRE investment
Financing Receivable, Past Due [Line Items]
Current	446,925	442,145
Total	447,900	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	975	1,073
Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Past Due [Line Items]
Current	106,548	90,887
Total	107,277	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	196	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	50,783	54,101
Total	51,332	54,403
Residential | Residential construction | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	549	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	416,273	428,341
Total	417,694	432,167
Residential | Residential first mortgage | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	354	2,736
Residential | Residential first mortgage | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	1,067	1,090
Residential | Residential junior mortgage
Financing Receivable, Past Due [Line Items]
Current	113,758	122,074
Total	114,323	122,771
Residential | Residential junior mortgage | 30-89 Days Past Due (accruing)
Financing Receivable, Past Due [Line Items]
Past due loans	0	217
Residential | Residential junior mortgage | 90 Days & Over or nonaccrual
Financing Receivable, Past Due [Line Items]
Past due loans	$ 565	$ 480