Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Unpaid Principal Balance
Impaired loans with no related allowance	$ 19,511		$ 19,511		$ 25,979
Impaired loans with related allowance	2,201		2,201		6,123
Total Impaired Loans	21,712		21,712		32,102
Recorded Investment
Impaired loans with no related allowance	11,962		11,962		17,930
Impaired loans with related allowance	2,114		2,114		5,281
Total Impaired Loans	14,076		14,076		23,211
Related Allowance	497		497		1,637
Average Recorded Investment
Impaired loans with no related allowance	12,758	$ 22,266	13,128	$ 22,709
Impaired loans with related allowance	2,124	5,432	2,137	5,442
Total Impaired Loans	14,882	27,698	15,265	28,151
Interest Income Recognized
Impaired loans with no related allowance	41	90	89	177
Impaired loans with related allowance	14	8	29	15
Total Impaired Loans	55	98	118	192
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	6,145		6,145		7,611
Impaired loans with related allowance	108		108		812
Recorded Investment
Impaired loans with no related allowance	999		999		1,536
Impaired loans with related allowance	108		108		782
Related Allowance	76		76		552
Average Recorded Investment
Impaired loans with no related allowance	1,004	1,053	1,006	1,211
Impaired loans with related allowance	108		108
Interest Income Recognized
Impaired loans with no related allowance		2		2
Impaired loans with related allowance
Agricultural production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	660		660		732
Impaired loans with related allowance					357
Recorded Investment
Impaired loans with no related allowance	640		640		700
Impaired loans with related allowance					327
Related Allowance					114
Average Recorded Investment
Impaired loans with no related allowance	640	660	640	660
Interest Income Recognized
Impaired loans with no related allowance
Real estate loans - Construction
Unpaid Principal Balance
Impaired loans with no related allowance	1,352		1,352
Recorded Investment
Impaired loans with no related allowance	614		614
Average Recorded Investment
Impaired loans with no related allowance	1,106		1,167
Interest Income Recognized
Impaired loans with no related allowance
Real estate loans - Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	11,262		11,262		16,758
Impaired loans with related allowance					2,989
Recorded Investment
Impaired loans with no related allowance	9,636		9,636		15,162
Impaired loans with related allowance					2,270
Related Allowance					567
Average Recorded Investment
Impaired loans with no related allowance	9,935	20,339	10,241	20,623
Impaired loans with related allowance		3,282		3,288
Interest Income Recognized
Impaired loans with no related allowance	40	88	87	175
Impaired loans with related allowance
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	82		82		833
Impaired loans with related allowance	1,773		1,773		1,616
Recorded Investment
Impaired loans with no related allowance	63		63		519
Impaired loans with related allowance	1,706		1,706		1,572
Related Allowance	348		348		327
Average Recorded Investment
Impaired loans with no related allowance	63	207	63	207
Impaired loans with related allowance	1,714	1,871	1,726	1,873
Interest Income Recognized
Impaired loans with no related allowance	1		2
Impaired loans with related allowance	12	6	24	11
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance					40
Recorded Investment
Impaired loans with no related allowance					8
Average Recorded Investment
Impaired loans with related allowance	302	279	303	281
Interest Income Recognized
Impaired loans with related allowance	2	2	5	4
Individuals' loans for household and other personal expenditures
Unpaid Principal Balance
Impaired loans with no related allowance	10		10		5
Recorded Investment
Impaired loans with no related allowance	10		10		5
Average Recorded Investment
Impaired loans with no related allowance	10	7	11	$ 8
Interest Income Recognized
Impaired loans with no related allowance
Home equity
Unpaid Principal Balance
Impaired loans with related allowance	320		320		349
Recorded Investment
Impaired loans with related allowance	300		300		330
Related Allowance	$ 73		$ 73		$ 77