Loans, Primary Segments Portfolio and Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	$ 1,090,917		$ 1,081,883
Individually evaluated for impairment	20,964		23,450
Loans acquired with deteriorated credit quality	1,805		1,859
Collectively evaluated for impairment	1,068,148		1,056,574
Allowance for loan losses	13,084		12,884
Individually evaluated for impairment	721		676
Allowance for loan losses related to loans acquired with deteriorated credit quality	0		0
Collectively evaluated for impairment	12,363		12,208
Net loans	1,077,833		1,068,999
Net loans, individually evaluated for impairment	20,243		22,774
Net loans, loans acquired with deteriorated credit quality	1,805		1,859
Net loans, collectively evaluated for impairment	$ 1,055,785		1,044,366
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	$ 214,635		215,305
Individually evaluated for impairment	1,178		890
Loans acquired with deteriorated credit quality	28		28
Collectively evaluated for impairment	213,429		214,387
Individually evaluated for impairment	23		24
Collectively evaluated for impairment	1,066		1,081
Real Estate Loans [Member] | Commercial [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	334,371		319,265
Individually evaluated for impairment	10,724		13,327
Loans acquired with deteriorated credit quality	1,291		1,321
Collectively evaluated for impairment	322,356		304,617
Individually evaluated for impairment	310		216
Collectively evaluated for impairment	3,820		3,899
Real Estate Loans [Member] | Agricultural [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	295,547		284,520
Individually evaluated for impairment	5,562		5,592
Loans acquired with deteriorated credit quality	0		0
Collectively evaluated for impairment	289,985		278,928
Individually evaluated for impairment	83		84
Collectively evaluated for impairment	4,309		4,180
Real Estate Loans [Member] | Construction [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	18,611		33,913
Individually evaluated for impairment	0		0
Loans acquired with deteriorated credit quality	0		0
Collectively evaluated for impairment	18,611		33,913
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	32		58
Consumer [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	9,773		9,858
Individually evaluated for impairment	0		0
Loans acquired with deteriorated credit quality	0		0
Collectively evaluated for impairment	9,773		9,858
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	124		120
Other Commercial Loans [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	74,323		74,118
Individually evaluated for impairment	2,072		2,206
Loans acquired with deteriorated credit quality	486		510
Collectively evaluated for impairment	71,765		71,402
Individually evaluated for impairment	146		193
Collectively evaluated for impairment	1,137		1,161
Other Agricultural Loans [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	43,245		42,186
Individually evaluated for impairment	1,428		1,435
Loans acquired with deteriorated credit quality	0		0
Collectively evaluated for impairment	41,817		40,751
Individually evaluated for impairment	159		159
Collectively evaluated for impairment	597		593
State and Political Subdivision Loans [Member]
Segments of loan portfolio and allowance for loan losses [Abstract]
Total financing receivables	100,412		102,718
Individually evaluated for impairment	0		0
Loans acquired with deteriorated credit quality	0		0
Collectively evaluated for impairment	100,412		102,718
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	565		762
First National Bank of Fredericksburg [Member]
Changes in the accretable yield for purchased credit-impaired loans [Roll Forward]
Balance at beginning of period	104	$ 106
Accretion	(2)	(24)
Balance at end of period	102	$ 82
Loans acquired with specific evidence of deterioration in credit quality [Abstract]
Outstanding balance	4,521		4,529
Carrying amount	$ 1,805		$ 1,859