Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Unpaid Principal Balance
Impaired loans with no related allowance	$ 76,952		$ 76,952		$ 100,416
Impaired loans with related allowance	6,258		6,258		4,963
Total Impaired Loans	83,210		83,210		105,379
Recorded Investment
Impaired loans with no related allowance	48,999		48,999		68,371
Impaired loans with related allowance	5,480		5,480		3,679
Total Impaired Loans	54,479		54,479		72,050
Related Allowance	1,399		1,399		612
Average Recorded Investment
Impaired loans with no related allowance	51,820	$ 81,215	58,009	$ 83,884
Impaired loans with related allowance	5,492	3,517	5,510	3,640
Total Impaired Loans	57,312	84,732	63,519	87,524
Interest Income Recognized
Impaired loans with no related allowance	598	1,058	1,553	3,173
Impaired loans with related allowance		6	4	6
Total Impaired Loans	598	1,064	1,557	3,179
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	15,443		15,443		17,645
Impaired loans with related allowance	1,824		1,824
Recorded Investment
Impaired loans with no related allowance	4,526		4,526		10,074
Impaired loans with related allowance	1,796		1,796
Related Allowance	206		206
Average Recorded Investment
Impaired loans with no related allowance	4,668	10,025	8,821	10,436
Impaired loans with related allowance	1,796	469	1,796	471
Interest Income Recognized
Impaired loans with no related allowance	56	89	149	261
Impaired loans with related allowance
Agricultural production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	1,818		1,818		757
Impaired loans with related allowance	357		357		660
Recorded Investment
Impaired loans with no related allowance	1,489		1,489		680
Impaired loans with related allowance	337		337		660
Related Allowance	270		270		36
Average Recorded Investment
Impaired loans with no related allowance	1,489	636	1,491	644
Impaired loans with related allowance	337	1,164	337	1,236
Interest Income Recognized
Impaired loans with no related allowance	9	3	9	3
Impaired loans with related allowance		6		6
Real estate loans - Construction
Unpaid Principal Balance
Impaired loans with no related allowance	2,583		2,583		5,946
Recorded Investment
Impaired loans with no related allowance	1,422		1,422		3,178
Average Recorded Investment
Impaired loans with no related allowance	1,427	3,526	1,438	3,572
Interest Income Recognized
Impaired loans with no related allowance	26	72	36	215
Real estate loans - Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	51,302		51,302		67,936
Impaired loans with related allowance	4,077		4,077		4,238
Recorded Investment
Impaired loans with no related allowance	38,098		38,098		49,731
Impaired loans with related allowance	3,347		3,347		2,985
Related Allowance	923		923		553
Average Recorded Investment
Impaired loans with no related allowance	40,668	61,903	42,459	63,722
Impaired loans with related allowance	3,359	1,850	3,377	1,894
Interest Income Recognized
Impaired loans with no related allowance	475	854	1,266	2,576
Impaired loans with related allowance			4
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	5,611		5,611		8,039
Impaired loans with related allowance					65
Recorded Investment
Impaired loans with no related allowance	3,310		3,310		4,664
Impaired loans with related allowance					34
Related Allowance					23
Average Recorded Investment
Impaired loans with no related allowance	3,410	5,081	3,637	5,453
Impaired loans with related allowance		34		39
Interest Income Recognized
Impaired loans with no related allowance	31	40	92	118
Impaired loans with related allowance
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	195		195		82
Recorded Investment
Impaired loans with no related allowance	154		154		44
Average Recorded Investment
Impaired loans with no related allowance	158	44	163	57
Interest Income Recognized
Impaired loans with no related allowance	$ 1		$ 1
Other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance					11
Recorded Investment
Impaired loans with no related allowance