Loans, Allowance for Loan Losses, and Credit Quality - Summary of loans by past due status (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 2,232,752	$ 2,159,139
Total	$ 2,242,931	$ 2,166,181
Percent of current loans	99.60%	99.70%
Percent of total loans	100.00%	100.00%
30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	$ 941	$ 1,571
Percent past due	0.00%	0.10%
90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	$ 9,238	$ 5,471
Percent past due	0.40%	0.20%
Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 28,581	$ 26,483
Total	28,713	26,493
Retail & other | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	124	10
Retail & other | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	8	0
Commercial portfolio segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	761,248	682,104
Total	763,742	684,920
Commercial portfolio segment | Commercial & industrial | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	215	0
Commercial portfolio segment | Commercial & industrial | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	2,279	2,816
Commercial portfolio segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	454,206	440,123
Total	456,508	441,353
Commercial portfolio segment | Owner-occupied CRE | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	557
Commercial portfolio segment | Owner-occupied CRE | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	2,302	673
Commercial portfolio segment | AG production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	34,799	35,606
Total	36,050	35,625
Commercial portfolio segment | AG production | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	19
Commercial portfolio segment | AG production | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	1,251	0
Commercial real estate portfolio segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	57,745	53,245
Total	58,591	53,444
Commercial real estate portfolio segment | AG real estate | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	35
Commercial real estate portfolio segment | AG real estate | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	846	164
Commercial real estate portfolio segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	335,331	343,262
Total	336,442	343,652
Commercial real estate portfolio segment | CRE investment | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	180
Commercial real estate portfolio segment | CRE investment | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	1,111	210
Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	61,810	80,519
Total	61,810	80,599
Commercial real estate portfolio segment | Construction & land development | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	0
Commercial real estate portfolio segment | Construction & land development | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	80
Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	342,216	355,818
Total	343,400	357,841
Residential | Residential first mortgage | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	319	758
Residential | Residential first mortgage | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	865	1,265
Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	115,320	111,054
Total	116,179	111,328
Residential | Residential junior mortgage | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	283	12
Residential | Residential junior mortgage | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	576	262
Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	41,496	30,925
Total	41,496	30,926
Residential | Residential construction | 30-89 Days Past Due (accruing)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	0	0
Residential | Residential construction | 90 Days & Over or nonaccrual
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	$ 0	$ 1