Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Unpaid Principal Balance
Impaired loans with no related allowance	$ 26,301		$ 25,979
Impaired loans with related allowance	4,670		6,123
Total Impaired Loans	30,971		32,102
Recorded Investment
Impaired loans with no related allowance	17,649		17,930
Impaired loans with related allowance	4,515		5,281
Total Impaired Loans	22,164		23,211
Related Allowance	1,317		1,637
Average Recorded Investment
Impaired loans with no related allowance	17,714	$ 16,997
Impaired loans with related allowance	4,598	4,827
Total Impaired Loans	22,312	21,824
Interest Income Recognized
Impaired loans with no related allowance	54	81
Impaired loans with related allowance	14	8
Total Impaired Loans	68	89
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	7,863		7,611
Impaired loans with related allowance	474		812
Recorded Investment
Impaired loans with no related allowance	1,812		1,536
Impaired loans with related allowance	462		782
Related Allowance	143		552
Average Recorded Investment
Impaired loans with no related allowance	1,845	1,066
Impaired loans with related allowance	465
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		732
Impaired loans with related allowance			357
Recorded Investment
Impaired loans with no related allowance	640		700
Impaired loans with related allowance			327
Related Allowance			114
Average Recorded Investment
Impaired loans with no related allowance	640
Impaired loans with related allowance		657
Interest Income Recognized
Impaired loans with no related allowance
Impaired loans with related allowance
Real estate loans - Construction
Unpaid Principal Balance
Impaired loans with related allowance	1,352
Recorded Investment
Impaired loans with related allowance	1,352
Related Allowance	307
Average Recorded Investment
Impaired loans with related allowance	1,352
Real estate loans - Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	16,581		16,758
Impaired loans with related allowance	823		2,989
Recorded Investment
Impaired loans with no related allowance	14,388		15,162
Impaired loans with related allowance	761		2,270
Related Allowance	451		567
Average Recorded Investment
Impaired loans with no related allowance	14,450	14,963
Impaired loans with related allowance	767	2,848
Interest Income Recognized
Impaired loans with no related allowance	48	80
Impaired loans with related allowance
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	1,146		833
Impaired loans with related allowance	1,697		1,616
Recorded Investment
Impaired loans with no related allowance	790		519
Impaired loans with related allowance	1,636		1,572
Related Allowance	341		327
Average Recorded Investment
Impaired loans with no related allowance	759	935
Impaired loans with related allowance	1,709	1,062
Interest Income Recognized
Impaired loans with no related allowance	6	1
Impaired loans with related allowance	12	6
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	40		40
Recorded Investment
Impaired loans with no related allowance	8		8
Average Recorded Investment
Impaired loans with no related allowance	8	26
Impaired loans with related allowance	305	260
Interest Income Recognized
Impaired loans with no related allowance
Impaired loans with related allowance	2	2
Individuals' loans for household and other personal expenditures
Unpaid Principal Balance
Impaired loans with no related allowance	11		5
Recorded Investment
Impaired loans with no related allowance	11		5
Average Recorded Investment
Impaired loans with no related allowance	12	7
Interest Income Recognized
Impaired loans with no related allowance
Home equity
Unpaid Principal Balance
Impaired loans with related allowance	324		349
Recorded Investment
Impaired loans with related allowance	304		330
Related Allowance	$ 75		$ 77