Loans, Allowance for Loan Losses, and Credit Quality - Nonaccrual loans by portfolio segment (Details 4) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 10,182	$ 13,095
% of Total	100.00%	100.00%
Percent of total loans	0.50%	0.60%
Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 4
% of Total	0.00%	0.00%
Commercial Portfolio Segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 5,803	$ 6,016
% of Total	57.00%	46.00%
Commercial Portfolio Segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 474	$ 533
% of Total	4.60%	4.10%
Commercial Portfolio Segment | AG Production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
% of Total	0.00%	0.00%
Commercial Real Estate Portfolio Segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 175	$ 186
% of Total	1.70%	1.40%
Commercial Real Estate Portfolio Segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,381	$ 4,531
% of Total	13.60%	34.60%
Commercial Real Estate Portfolio Segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 80	$ 0
% of Total	0.80%	0.00%
Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,973	$ 1,587
% of Total	19.40%	12.10%
Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 268	$ 158
% of Total	2.60%	1.20%
Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 28	$ 80
% of Total	0.30%	0.60%
Originated
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 4,878	$ 3,278
% of Total	100.00%	100.00%
Percent of total loans	47.90%	25.00%
Originated | Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 4
% of Total	0.00%	0.10%
Originated | Commercial Portfolio Segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 2,729	$ 2,296
% of Total	55.90%	70.00%
Originated | Commercial Portfolio Segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 360	$ 86
% of Total	7.40%	2.60%
Originated | Commercial Portfolio Segment | AG Production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
% of Total	0.00%	0.00%
Originated | Commercial Real Estate Portfolio Segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
% of Total	0.00%	0.00%
Originated | Commercial Real Estate Portfolio Segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 918	$ 549
% of Total	18.80%	16.80%
Originated | Commercial Real Estate Portfolio Segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
% of Total	0.00%	0.00%
Originated | Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 777	$ 331
% of Total	15.90%	10.10%
Originated | Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 66	$ 12
% of Total	1.40%	0.40%
Originated | Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 28	$ 0
% of Total	0.60%	0.00%
Acquired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 5,304	$ 9,817
% of Total	100.00%	100.00%
Percent of total loans	52.10%	75.00%
Acquired | Retail & other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
% of Total	0.00%	0.00%
Acquired | Commercial Portfolio Segment | Commercial & industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 3,074	$ 3,720
% of Total	58.00%	37.90%
Acquired | Commercial Portfolio Segment | Owner-occupied CRE
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 114	$ 447
% of Total	2.20%	4.60%
Acquired | Commercial Portfolio Segment | AG Production
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 0
% of Total	0.00%	0.00%
Acquired | Commercial Real Estate Portfolio Segment | AG real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 175	$ 186
% of Total	3.30%	1.90%
Acquired | Commercial Real Estate Portfolio Segment | CRE investment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 463	$ 3,982
% of Total	8.70%	40.60%
Acquired | Commercial Real Estate Portfolio Segment | Construction & land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 80	$ 0
% of Total	1.50%	0.00%
Acquired | Residential | Residential first mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 1,196	$ 1,256
% of Total	22.50%	12.80%
Acquired | Residential | Residential junior mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 202	$ 146
% of Total	3.80%	1.40%
Acquired | Residential | Residential construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 0	$ 80
% of Total	0.00%	0.80%