Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unpaid Principal Balance
Impaired loans with no related allowance	$ 9,955	$ 14,555	$ 25,979
Impaired loans with related allowance	5,139	10,501	6,123
Total Impaired Loans	15,094	25,056	32,102
Recorded Investment
Impaired loans with no related allowance	7,352	11,594	17,931
Impaired loans with related allowance	4,357	10,431	5,280
Total Impaired Loans	11,709	22,025	23,211
Related Allowance	689	1,872	1,637
Average Recorded Investment
Impaired loans with no related allowance	7,924	15,824	22,752
Impaired loans with related allowance	4,405	10,448	6,135
Total Impaired Loans	12,329	26,272	28,887
Interest Income Recognized
Impaired loans with no related allowance	159	168	360
Impaired loans with related allowance	75	68	39
Total Impaired Loans	234	236	399
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	320	828	7,611
Impaired loans with related allowance			812
Recorded Investment
Impaired loans with no related allowance	320	806	1,536
Impaired loans with related allowance			782
Related Allowance			552
Average Recorded Investment
Impaired loans with no related allowance	320	833	3,839
Impaired loans with related allowance			1,517
Interest Income Recognized
Impaired loans with no related allowance	0	0	0
Impaired loans with related allowance			0
Agriculture production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	299	679	732
Impaired loans with related allowance			357
Recorded Investment
Impaired loans with no related allowance	137	679	700
Impaired loans with related allowance			327
Related Allowance			114
Average Recorded Investment
Impaired loans with no related allowance	298	679	762
Impaired loans with related allowance			327
Interest Income Recognized
Impaired loans with no related allowance	0	0	0
Impaired loans with related allowance			0
Construction
Unpaid Principal Balance
Impaired loans with no related allowance	1,206	1,352
Impaired loans with related allowance		7,978
Recorded Investment
Impaired loans with no related allowance	970	614
Impaired loans with related allowance		7,977
Related Allowance		1,429
Average Recorded Investment
Impaired loans with no related allowance	1,229	835
Impaired loans with related allowance		7,977
Interest Income Recognized
Impaired loans with no related allowance	0	0
Impaired loans with related allowance		0
Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	8,037	11,176	16,758
Impaired loans with related allowance	2,648	171	2,988
Recorded Investment
Impaired loans with no related allowance	5,849	8,994	15,163
Impaired loans with related allowance	1,909	171	2,269
Related Allowance	231	6	567
Average Recorded Investment
Impaired loans with no related allowance	6,000	12,975	17,495
Impaired loans with related allowance	1,909	171	2,379
Interest Income Recognized
Impaired loans with no related allowance	156	165	360
Impaired loans with related allowance	0	0	0
Residential
Unpaid Principal Balance
Impaired loans with no related allowance	93	118	833
Impaired loans with related allowance	2,070	1,958	1,616
Recorded Investment
Impaired loans with no related allowance	76	100	519
Impaired loans with related allowance	2,044	1,907	1,572
Related Allowance	383	362	327
Average Recorded Investment
Impaired loans with no related allowance	77	101	635
Impaired loans with related allowance	2,083	1,915	1,580
Interest Income Recognized
Impaired loans with no related allowance	3	3	0
Impaired loans with related allowance	63	57	28
Home equity
Unpaid Principal Balance
Impaired loans with no related allowance		49	40
Impaired loans with related allowance	417	376	350
Recorded Investment
Impaired loans with no related allowance		48	8
Impaired loans with related allowance	400	358	330
Related Allowance	75	74	77
Average Recorded Investment
Impaired loans with no related allowance		48	14
Impaired loans with related allowance	409	365	332
Interest Income Recognized
Impaired loans with no related allowance		0	0
Impaired loans with related allowance	12	10	11
Individuals' loans for household and other personal expenditures
Unpaid Principal Balance
Impaired loans with no related allowance			5
Impaired loans with related allowance	4	18
Recorded Investment
Impaired loans with no related allowance			5
Impaired loans with related allowance	4	18
Related Allowance	0	1
Average Recorded Investment
Impaired loans with no related allowance			7
Impaired loans with related allowance	4	20
Interest Income Recognized
Impaired loans with no related allowance			$ 0
Impaired loans with related allowance	$ 0	1
Public finance and other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance		353
Recorded Investment
Impaired loans with no related allowance		353
Average Recorded Investment
Impaired loans with no related allowance		353
Interest Income Recognized
Impaired loans with no related allowance		$ 0