Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unpaid Principal Balance
Impaired loans with no related allowance	$ 75,547	$ 100,416	$ 134,581
Impaired loans with related allowance	4,158	4,963	7,953
Total Impaired Loans	79,705	105,379	142,534
Recorded Investment
Impaired loans with no related allowance	49,308	68,371	90,975
Impaired loans with related allowance	3,379	3,679	6,552
Total Impaired Loans	52,687	72,050	97,527
Related Allowance	1,233	612	1,842
Average Recorded Investment
Impaired loans with no related allowance	57,428	74,592	96,423
Impaired loans with related allowance	4,227	3,718	6,945
Total Impaired Loans	61,655	78,310	103,368
Interest Income Recognized
Impaired loans with no related allowance	2,126	3,905	4,217
Impaired loans with related allowance	4		38
Total Impaired Loans	2,130	3,905	4,255
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	14,110	17,645	22,151
Impaired loans with related allowance	812		3,043
Recorded Investment
Impaired loans with no related allowance	3,850	10,074	11,669
Impaired loans with related allowance	782		2,690
Related Allowance	552		1,247
Average Recorded Investment
Impaired loans with no related allowance	7,653	13,393	12,578
Impaired loans with related allowance	1,517		2,752
Interest Income Recognized
Impaired loans with no related allowance	186	499	488
Impaired loans with related allowance			38
Agriculture production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	1,595	757	370
Impaired loans with related allowance	357	660	466
Recorded Investment
Impaired loans with no related allowance	1,238	680	361
Impaired loans with related allowance	327	660	466
Related Allowance	114	36	30
Average Recorded Investment
Impaired loans with no related allowance	1,304	750	439
Impaired loans with related allowance	327	691	538
Interest Income Recognized
Impaired loans with no related allowance	20	3
Impaired loans with related allowance
Construction
Unpaid Principal Balance
Impaired loans with no related allowance	2,530	5,946	4,551
Recorded Investment
Impaired loans with no related allowance	1,396	3,178	2,336
Average Recorded Investment
Impaired loans with no related allowance	1,420	3,255	3,662
Interest Income Recognized
Impaired loans with no related allowance	64	258	157
Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	51,829	67,936	95,930
Impaired loans with related allowance	2,989	4,238	2,144
Recorded Investment
Impaired loans with no related allowance	39,651	49,731	69,024
Impaired loans with related allowance	2,270	2,985	1,933
Related Allowance	567	553	392
Average Recorded Investment
Impaired loans with no related allowance	43,445	51,740	71,569
Impaired loans with related allowance	2,383	2,989	1,868
Interest Income Recognized
Impaired loans with no related allowance	1,734	2,978	3,328
Impaired loans with related allowance	4
Residential
Unpaid Principal Balance
Impaired loans with no related allowance	5,403	8,039	11,262
Impaired loans with related allowance		65	2,300
Recorded Investment
Impaired loans with no related allowance	3,133	4,664	7,338
Impaired loans with related allowance		34	1,463
Related Allowance		23	173
Average Recorded Investment
Impaired loans with no related allowance	3,558	5,410	7,926
Impaired loans with related allowance		38	1,787
Interest Income Recognized
Impaired loans with no related allowance	122	167	244
Impaired loans with related allowance
Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	80	82	297
Recorded Investment
Impaired loans with no related allowance	40	44	247
Average Recorded Investment
Impaired loans with no related allowance	48	44	249
Interest Income Recognized
Impaired loans with no related allowance
Other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance		11	20
Recorded Investment
Impaired loans with no related allowance
Average Recorded Investment
Impaired loans with no related allowance
Interest Income Recognized
Impaired loans with no related allowance