Loans, Allowance for Loan Losses, and Credit Quality - Summary of changes in ALLL by portfolio segment - As percent of ALLL (Details 3) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
ALLL:
Individually evaluated	$ 150	$ 163
Collectively evaluated	12,615	12,490
Ending balance	12,765	12,653	$ 12,189	$ 11,820
Loans:
Individually evaluated	15,480	17,221
Collectively evaluated	2,085,117	2,070,704
Total loans	2,100,597	2,087,925
Less ALLL	12,765	12,653	$ 12,189	11,820
Net loans	2,087,832	2,075,272
Retail & other
ALLL:
Individually evaluated	0	0
Collectively evaluated	244	225
Ending balance	244	225		152
Loans:
Individually evaluated	12	12
Collectively evaluated	22,511	22,803
Total loans	22,523	22,815
Less ALLL	244	225		152
Net loans	22,279	22,590
Commercial Portfolio Segment | Commercial & industrial
ALLL:
Individually evaluated	115	163
Collectively evaluated	5,022	4,771
Ending balance	5,137	4,934		3,919
Loans:
Individually evaluated	6,273	5,870
Collectively evaluated	643,924	631,467
Total loans	650,197	637,337
Less ALLL	5,137	4,934		3,919
Net loans	645,060	632,403
Commercial Portfolio Segment | Owner-occupied CRE
ALLL:
Individually evaluated	0	0
Collectively evaluated	2,611	2,607
Ending balance	2,611	2,607
Loans:
Individually evaluated	1,886	1,689
Collectively evaluated	435,786	428,354
Total loans	437,672	430,043
Less ALLL	2,611	2,607
Net loans	435,061	427,436
Commercial Portfolio Segment | AG Production
ALLL:
Individually evaluated	0	0
Collectively evaluated	121	129
Ending balance	121	129		150
Loans:
Individually evaluated	0	0
Collectively evaluated	33,741	35,455
Total loans	33,741	35,455
Less ALLL	121	129		150
Net loans	33,620	35,326
Commercial Real Estate Portfolio Segment | AG real estate
ALLL:
Individually evaluated	0	0
Collectively evaluated	312	296
Ending balance	312	296		285
Loans:
Individually evaluated	235	248
Collectively evaluated	53,177	51,530
Total loans	53,412	51,778
Less ALLL	312	296		285
Net loans	53,100	51,482
Commercial Real Estate Portfolio Segment | CRE investment
ALLL:
Individually evaluated	35	0
Collectively evaluated	1,486	1,388
Ending balance	1,521	1,388		1,124
Loans:
Individually evaluated	2,798	5,290
Collectively evaluated	320,250	309,173
Total loans	323,048	314,463
Less ALLL	1,521	1,388		1,124
Net loans	321,527	313,075
Commercial Real Estate Portfolio Segment | Construction & land development
ALLL:
Individually evaluated	0	0
Collectively evaluated	494	726
Ending balance	494	726		774
Loans:
Individually evaluated	1,049	1,053
Collectively evaluated	76,659	88,607
Total loans	77,708	89,660
Less ALLL	494	726		774
Net loans	77,214	88,934
Residential | First mortgage
ALLL:
Individually evaluated	0	0
Collectively evaluated	1,628	1,609
Ending balance	1,628	1,609		1,784
Loans:
Individually evaluated	3,060	2,801
Collectively evaluated	362,258	360,551
Total loans	365,318	363,352
Less ALLL	1,628	1,609		1,784
Net loans	363,690	361,743
Residential | Junior mortgage
ALLL:
Individually evaluated	0	0
Collectively evaluated	485	488
Ending balance	485	488		461
Loans:
Individually evaluated	87	178
Collectively evaluated	105,816	105,849
Total loans	105,903	106,027
Less ALLL	485	488		461
Net loans	105,418	105,539
Residential | Residential construction
ALLL:
Individually evaluated	0	0
Collectively evaluated	212	251
Ending balance	212	251		304
Loans:
Individually evaluated	80	80
Collectively evaluated	30,995	36,915
Total loans	31,075	36,995
Less ALLL	212	251		304
Net loans	30,863	36,744
Originated
ALLL:
Individually evaluated	150	163
Collectively evaluated	10,549	10,379
Ending balance	10,699	10,542		9,449
Loans:
Individually evaluated	5,062	3,003
Collectively evaluated	1,289,686	1,240,699
Total loans	1,294,748	1,243,702
Less ALLL	10,699	10,542		9,449
Net loans	1,284,049	1,233,160
Originated | Retail & other
ALLL:
Individually evaluated	0	0
Collectively evaluated	219	200
Ending balance	219	200		132
Loans:
Individually evaluated	0	0
Collectively evaluated	18,611	18,254
Total loans	18,611	18,254
Less ALLL	219	200		132
Net loans	18,392	18,054
Originated | Commercial Portfolio Segment | Commercial & industrial
ALLL:
Individually evaluated	115	163
Collectively evaluated	4,299	4,029
Ending balance	4,414	4,192		3,150
Loans:
Individually evaluated	2,686	2,189
Collectively evaluated	504,774	486,411
Total loans	507,460	488,600
Less ALLL	4,414	4,192		3,150
Net loans	503,046	484,408
Originated | Commercial Portfolio Segment | Owner-occupied CRE
ALLL:
Individually evaluated	0	0
Collectively evaluated	2,131	2,115
Ending balance	2,131	2,115
Loans:
Individually evaluated	343	0
Collectively evaluated	246,706	237,548
Total loans	247,049	237,548
Less ALLL	2,131	2,115
Net loans	244,918	235,433
Originated | Commercial Portfolio Segment | AG Production
ALLL:
Individually evaluated	0	0
Collectively evaluated	106	112
Ending balance	106	112		122
Loans:
Individually evaluated	0	0
Collectively evaluated	10,515	11,102
Total loans	10,515	11,102
Less ALLL	106	112		122
Net loans	10,409	10,990
Originated | Commercial Real Estate Portfolio Segment | AG real estate
ALLL:
Individually evaluated	0	0
Collectively evaluated	251	235
Ending balance	251	235		222
Loans:
Individually evaluated	0	0
Collectively evaluated	30,014	27,831
Total loans	30,014	27,831
Less ALLL	251	235		222
Net loans	29,763	27,596
Originated | Commercial Real Estate Portfolio Segment | CRE investment
ALLL:
Individually evaluated	35	0
Collectively evaluated	1,235	1,154
Ending balance	1,270	1,154		893
Loans:
Individually evaluated	1,522	549
Collectively evaluated	129,904	113,313
Total loans	131,426	113,862
Less ALLL	1,270	1,154		893
Net loans	130,156	112,708
Originated | Commercial Real Estate Portfolio Segment | Construction & land development
ALLL:
Individually evaluated	0	0
Collectively evaluated	412	628
Ending balance	412	628		656
Loans:
Individually evaluated	0	0
Collectively evaluated	50,348	56,061
Total loans	50,348	56,061
Less ALLL	412	628		656
Net loans	49,936	55,433
Originated | Residential | First mortgage
ALLL:
Individually evaluated	0	0
Collectively evaluated	1,317	1,297
Ending balance	1,317	1,297		1,372
Loans:
Individually evaluated	511	253
Collectively evaluated	202,056	190,933
Total loans	202,567	191,186
Less ALLL	1,317	1,297		1,372
Net loans	201,250	189,889
Originated | Residential | Junior mortgage
ALLL:
Individually evaluated	0	0
Collectively evaluated	409	409
Ending balance	409	409		373
Loans:
Individually evaluated	0	12
Collectively evaluated	67,654	65,631
Total loans	67,654	65,643
Less ALLL	409	409		373
Net loans	67,245	65,234
Originated | Residential | Residential construction
ALLL:
Individually evaluated	0	0
Collectively evaluated	170	200
Ending balance	170	200		266
Loans:
Individually evaluated	0	0
Collectively evaluated	29,104	33,615
Total loans	29,104	33,615
Less ALLL	170	200		266
Net loans	28,934	33,415
Acquired
ALLL:
Individually evaluated	0	0
Collectively evaluated	2,066	2,111
Ending balance	2,066	2,111		2,371
Loans:
Individually evaluated	10,418	14,218
Collectively evaluated	795,431	830,005
Total loans	805,849	844,223
Less ALLL	2,066	2,111		2,371
Net loans	803,783	842,112
Acquired | Retail & other
ALLL:
Individually evaluated	0	0
Collectively evaluated	25	25
Ending balance	25	25		20
Loans:
Individually evaluated	12	12
Collectively evaluated	3,900	4,549
Total loans	3,912	4,561
Less ALLL	25	25		20
Net loans	3,887	4,536
Acquired | Commercial Portfolio Segment | Commercial & industrial
ALLL:
Individually evaluated	0	0
Collectively evaluated	723	742
Ending balance	723	742		769
Loans:
Individually evaluated	3,587	3,681
Collectively evaluated	139,150	145,056
Total loans	142,737	148,737
Less ALLL	723	742		769
Net loans	142,014	147,995
Acquired | Commercial Portfolio Segment | Owner-occupied CRE
ALLL:
Individually evaluated	0	0
Collectively evaluated	480	492
Ending balance	480	492
Loans:
Individually evaluated	1,543	1,689
Collectively evaluated	189,080	190,806
Total loans	190,623	192,495
Less ALLL	480	492
Net loans	190,143	192,003
Acquired | Commercial Portfolio Segment | AG Production
ALLL:
Individually evaluated	0	0
Collectively evaluated	15	17
Ending balance	15	17		28
Loans:
Individually evaluated	0	0
Collectively evaluated	23,226	24,353
Total loans	23,226	24,353
Less ALLL	15	17		28
Net loans	23,211	24,336
Acquired | Commercial Real Estate Portfolio Segment | AG real estate
ALLL:
Individually evaluated	0	0
Collectively evaluated	61	61
Ending balance	61	61		63
Loans:
Individually evaluated	235	248
Collectively evaluated	23,163	23,699
Total loans	23,398	23,947
Less ALLL	61	61		63
Net loans	23,337	23,886
Acquired | Commercial Real Estate Portfolio Segment | CRE investment
ALLL:
Individually evaluated	0	0
Collectively evaluated	251	234
Ending balance	251	234		231
Loans:
Individually evaluated	1,276	4,741
Collectively evaluated	190,346	195,860
Total loans	191,622	200,601
Less ALLL	251	234		231
Net loans	191,371	200,367
Acquired | Commercial Real Estate Portfolio Segment | Construction & land development
ALLL:
Individually evaluated	0	0
Collectively evaluated	82	98
Ending balance	82	98		118
Loans:
Individually evaluated	1,049	1,053
Collectively evaluated	26,311	32,546
Total loans	27,360	33,599
Less ALLL	82	98		118
Net loans	27,278	33,501
Acquired | Residential | First mortgage
ALLL:
Individually evaluated	0	0
Collectively evaluated	311	312
Ending balance	311	312		412
Loans:
Individually evaluated	2,549	2,548
Collectively evaluated	160,202	169,618
Total loans	162,751	172,166
Less ALLL	311	312		412
Net loans	162,440	171,854
Acquired | Residential | Junior mortgage
ALLL:
Individually evaluated	0	0
Collectively evaluated	76	79
Ending balance	76	79		88
Loans:
Individually evaluated	87	166
Collectively evaluated	38,162	40,218
Total loans	38,249	40,384
Less ALLL	76	79		88
Net loans	38,173	40,305
Acquired | Residential | Residential construction
ALLL:
Individually evaluated	0	0
Collectively evaluated	42	51
Ending balance	42	51		38
Loans:
Individually evaluated	80	80
Collectively evaluated	1,891	3,300
Total loans	1,971	3,380
Less ALLL	42	51		$ 38
Net loans	$ 1,929	$ 3,329