Loans, Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 11,941	$ 9,979	$ 11,190	$ 8,886
Charge-offs	(48)	(56)	(225)	(240)
Recoveries	15	24	118	61
Provision	475	500	1,300	1,740
Balance at end of period	12,383	10,447	12,383	10,447
Real Estate Loans [Member] | Residential [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	1,045	1,104	1,049	1,064
Charge-offs	(10)	(11)	(27)	(104)
Recoveries	1	0	70	0
Provision	16	(13)	(40)	120
Balance at end of period	1,052	1,080	1,052	1,080
Real Estate Loans [Member] | Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	3,794	3,541	3,867	3,589
Charge-offs	(25)	0	(25)	(41)
Recoveries	0	5	3	11
Provision	156	36	80	23
Balance at end of period	3,925	3,582	3,925	3,582
Real Estate Loans [Member] | Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	3,673	2,452	3,143	1,494
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	256	242	786	1,200
Balance at end of period	3,929	2,694	3,929	2,694
Real Estate Loans [Member] | Construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	44	45	23	47
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	5	(14)	26	(16)
Balance at end of period	49	31	49	31
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	115	125	124	122
Charge-offs	(13)	(45)	(32)	(90)
Recoveries	9	13	26	35
Provision	12	44	5	70
Balance at end of period	123	137	123	137
Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	1,266	1,131	1,272	1,327
Charge-offs	0	0	(91)	0
Recoveries	5	5	19	14
Provision	(52)	(24)	19	(229)
Balance at end of period	1,219	1,112	1,219	1,112
Other Agricultural Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	589	431	492	312
Charge-offs	0	0	(50)	(5)
Recoveries	0	1	0	1
Provision	110	30	257	154
Balance at end of period	699	462	699	462
State and Political Subdivision Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	767	838	816	833
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(16)	36	(65)	41
Balance at end of period	751	874	751	874
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	648	312	404	98
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(12)	163	232	377
Balance at end of period	$ 636	$ 475	$ 636	$ 475