Loans, Allowance for Loan Losses, and Credit Quality - Nonaccrual loans by portfolio segment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 9,238	$ 5,471
Percent of total	100.00%	100.00%
Percent of total loans	0.40%	0.20%
Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 8	$ 0
Percent of total	0.00%	0.00%
Commercial portfolio segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 2,279	$ 2,816
Percent of total	25.00%	52.00%
Commercial portfolio segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 2,302	$ 673
Percent of total	25.00%	12.00%
Commercial portfolio segment | AG production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,251	$ 0
Percent of total	14.00%	0.00%
Commercial real estate portfolio segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 846	$ 164
Percent of total	9.00%	3.00%
Commercial real estate portfolio segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,111	$ 210
Percent of total	12.00%	4.00%
Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 80
Percent of total	0.00%	1.00%
Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 865	$ 1,265
Percent of total	9.00%	23.00%
Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 576	$ 262
Percent of total	6.00%	5.00%
Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 1
Percent of total	0.00%	0.00%
Originated
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 5,076	$ 1,409
Percent of total	100.00%	100.00%
Percent of total loans	55.00%	26.00%
Originated | Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
Percent of total	0.00%	0.00%
Originated | Commercial portfolio segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 833	$ 352
Percent of total	16.00%	25.00%
Originated | Commercial portfolio segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,792	$ 362
Percent of total	35.00%	26.00%
Originated | Commercial portfolio segment | AG production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,095	$ 0
Percent of total	22.00%	0.00%
Originated | Commercial real estate portfolio segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 635	$ 0
Percent of total	13.00%	0.00%
Originated | Commercial real estate portfolio segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
Percent of total	0.00%	0.00%
Originated | Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
Percent of total	0.00%	0.00%
Originated | Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 458	$ 629
Percent of total	9.00%	45.00%
Originated | Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 263	$ 65
Percent of total	5.00%	4.00%
Originated | Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 1
Percent of total	0.00%	0.00%
Acquired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 4,162	$ 4,062
Percent of total	100.00%	100.00%
Percent of total loans	45.00%	74.00%
Acquired | Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 8	$ 0
Percent of total	0.00%	0.00%
Acquired | Commercial portfolio segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,446	$ 2,464
Percent of total	35.00%	61.00%
Acquired | Commercial portfolio segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 510	$ 311
Percent of total	12.00%	8.00%
Acquired | Commercial portfolio segment | AG production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 156	$ 0
Percent of total	4.00%	0.00%
Acquired | Commercial real estate portfolio segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 211	$ 164
Percent of total	5.00%	4.00%
Acquired | Commercial real estate portfolio segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,111	$ 210
Percent of total	27.00%	5.00%
Acquired | Commercial real estate portfolio segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 80
Percent of total	0.00%	2.00%
Acquired | Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 407	$ 636
Percent of total	10.00%	15.00%
Acquired | Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 313	$ 197
Percent of total	7.00%	5.00%
Acquired | Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
Percent of total	0.00%	0.00%