Loans, Primary Segments Portfolio and Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	$ 1,060,331		$ 1,060,331		$ 1,000,525
Individually evaluated for impairment	24,917		24,917		24,398
Loans acquired with deteriorated credit quality	2,576		2,576		2,638
Collectively evaluated for impairment	1,032,838		1,032,838		973,489
Allowance for loan losses	12,383		12,383		11,190
Individually evaluated for impairment	654		654		410
Allowance for loan losses related to loans acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	11,729		11,729		10,780
Net loans	1,047,948		1,047,948		989,335
Net loans, individually evaluated for impairment	24,263		24,263		23,988
Net loans, loans acquired with deteriorated credit quality	2,576		2,576		2,638
Net loans, collectively evaluated for impairment	1,021,109		$ 1,021,109		962,709
Minimum [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Number of days past for loan to be considered impaired, Minimum			90 days
First Mortgage [Member] | Minimum [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Period of mortgage on real estate			15 years
First Mortgage [Member] | Maximum [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Period of mortgage on real estate			30 years
Second Mortgage [Member] | Maximum [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Period of mortgage on real estate			15 years
Real Estate Loans [Member] | Residential [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	213,255		$ 213,255		214,479
Individually evaluated for impairment	915		915		1,065
Loans acquired with deteriorated credit quality	30		30		33
Collectively evaluated for impairment	212,310		212,310		213,381
Individually evaluated for impairment	27		27		56
Collectively evaluated for impairment	1,025		1,025		993
Real Estate Loans [Member] | Commercial [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	312,982		312,982		308,084
Individually evaluated for impairment	14,148		14,148		13,864
Loans acquired with deteriorated credit quality	1,345		1,345		1,460
Collectively evaluated for impairment	297,489		297,489		292,760
Individually evaluated for impairment	224		224		94
Collectively evaluated for impairment	3,701		3,701		3,773
Real Estate Loans [Member] | Agricultural [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	280,569		280,569		239,957
Individually evaluated for impairment	5,038		5,038		3,901
Loans acquired with deteriorated credit quality	683		683		702
Collectively evaluated for impairment	274,848		274,848		235,354
Individually evaluated for impairment	86		86		3
Collectively evaluated for impairment	3,843		3,843		3,140
Real Estate Loans [Member] | Construction [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	30,262		30,262		13,502
Individually evaluated for impairment	0		0		0
Loans acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	30,262		30,262		13,502
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	49		49		23
Consumer [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	9,702		9,702		9,944
Individually evaluated for impairment	0		0		8
Loans acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	9,702		9,702		9,936
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	123		123		124
Other Commercial Loans [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	72,219		72,219		72,013
Individually evaluated for impairment	3,368		3,368		4,197
Loans acquired with deteriorated credit quality	518		518		443
Collectively evaluated for impairment	68,333		68,333		67,373
Individually evaluated for impairment	152		152		231
Collectively evaluated for impairment	1,067		1,067		1,041
Other Agricultural Loans [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	39,917		39,917		37,809
Individually evaluated for impairment	1,448		1,448		1,363
Loans acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	38,469		38,469		36,446
Individually evaluated for impairment	165		165		26
Collectively evaluated for impairment	534		534		466
State and Political Subdivision Loans [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	101,425		101,425		104,737
Individually evaluated for impairment	0		0		0
Loans acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	101,425		101,425		104,737
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	751		751		816
First National Bank of Fredericksburg [Member]
Changes in the accretable yield for purchased credit-impaired loans [Roll Forward]
Balance at beginning of period	59	$ 167	106	$ 389
Accretion	(24)	(90)	(71)	(312)
Reclassification of non-accretable discount	93	0	93	0
Balance at end of period	128	$ 77	128	$ 77
Loans acquired with specific evidence of deterioration in credit quality [Abstract]
Outstanding balance	5,340		5,340		5,295
Carrying amount	$ 2,576		$ 2,576		$ 2,638