Loans and Allowance - Composition of Impaired Loans by Loan Class (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
Unpaid Principal Balance
Impaired loans with no related allowance	$ 21,951		$ 21,951		$ 25,979
Impaired loans with related allowance	2,231		2,231		6,123
Total Impaired Loans	24,182		24,182		32,102
Recorded Investment
Impaired loans with no related allowance	13,842		13,842		17,930
Impaired loans with related allowance	2,157		2,157		5,281
Total Impaired Loans	15,999		15,999		23,211
Related Allowance	433		433		1,637
Average Recorded Investment
Impaired loans with no related allowance	14,402	$ 17,782	15,370	$ 21,691
Impaired loans with related allowance	2,161	7,226	2,179	7,236
Total Impaired Loans	16,563	25,008	17,549	28,927
Interest Income Recognized
Impaired loans with no related allowance	41	89	130	269
Impaired loans with related allowance	16	10	45	31
Total Impaired Loans	57	99	175	300
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	6,133		6,133		7,611
Impaired loans with related allowance					812
Recorded Investment
Impaired loans with no related allowance	974		974		1,536
Impaired loans with related allowance					782
Related Allowance					552
Average Recorded Investment
Impaired loans with no related allowance	979	2,103	994	4,567
Impaired loans with related allowance		1,796		1,796
Interest Income Recognized
Impaired loans with no related allowance
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	945	640	945
Impaired loans with related allowance		337		337
Interest Income Recognized
Impaired loans with no related allowance
Impaired loans with related allowance
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	614		930
Interest Income Recognized
Impaired loans with no related allowance
Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	13,717		13,717		16,758
Impaired loans with related allowance					2,989
Recorded Investment
Impaired loans with no related allowance	11,544		11,544		15,162
Impaired loans with related allowance					2,270
Related Allowance					567
Average Recorded Investment
Impaired loans with no related allowance	12,098	14,129	12,733	15,483
Impaired loans with related allowance		3,359		3,374
Interest Income Recognized
Impaired loans with no related allowance	40	89	128	267
Impaired loans with related allowance
Residential
Unpaid Principal Balance
Impaired loans with no related allowance	81		81		833
Impaired loans with related allowance	1,849		1,849		1,616
Recorded Investment
Impaired loans with no related allowance	62		62		519
Impaired loans with related allowance	1,794		1,794		1,572
Related Allowance	357		357		327
Average Recorded Investment
Impaired loans with no related allowance	62	588	63	673
Impaired loans with related allowance	1,797	1,403	1,812	1,395
Interest Income Recognized
Impaired loans with no related allowance	1		2	2
Impaired loans with related allowance	13	8	37	25
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 no related allowance		11		16
Impaired loans with related allowance	364	331	367	334
Interest Income Recognized
Impaired loans with no related allowance
Impaired loans with related allowance	3	2	8	6
Individuals' loans for household and other personal expenditures
Unpaid Principal Balance
Impaired loans with no related allowance	8		8		5
Recorded Investment
Impaired loans with no related allowance	8		8		5
Average Recorded Investment
Impaired loans with no related allowance	9	6	10	$ 7
Interest Income Recognized
Impaired loans with no related allowance
Home equity
Unpaid Principal Balance
Impaired loans with related allowance	382		382		349
Recorded Investment
Impaired loans with related allowance	363		363		330
Related Allowance	$ 76		$ 76		$ 77