Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Unpaid Principal Balance
Impaired loans with no related allowance	$ 85,815		$ 100,416
Impaired loans with related allowance	4,411		4,963
Total Impaired Loans	90,226		105,379
Recorded Investment
Impaired loans with no related allowance	56,354		68,371
Impaired loans with related allowance	3,720		3,679
Total Impaired Loans	60,074		72,050
Related Allowance	888		612
Average Recorded Investment
Impaired loans with no related allowance	59,910	$ 90,462
Impaired loans with related allowance	3,726	5,313
Total Impaired Loans	63,636	95,775
Interest Income Recognized
Impaired loans with no related allowance	666	1,124
Impaired loans with related allowance		9
Total Impaired Loans	666	1,133
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	13,715		17,645
Recorded Investment
Impaired loans with no related allowance	5,786		10,074
Average Recorded Investment
Impaired loans with no related allowance	8,571	12,052
Impaired loans with related allowance		1,363
Interest Income Recognized
Impaired loans with no related allowance	48	109
Impaired loans with related allowance		9
Agricultural production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	14		757
Impaired loans with related allowance	660		660
Recorded Investment
Impaired loans with no related allowance	5		680
Impaired loans with related allowance	660		660
Related Allowance	36		36
Average Recorded Investment
Impaired loans with no related allowance	145	716
Impaired loans with related allowance	660	1,315
Interest Income Recognized
Impaired loans with no related allowance		1
Impaired loans with related allowance
Real estate loans - Construction
Unpaid Principal Balance
Impaired loans with no related allowance	589		5,946
Recorded Investment
Impaired loans with no related allowance			3,178
Average Recorded Investment
Impaired loans with no related allowance		4,262
Interest Income Recognized
Impaired loans with no related allowance		85
Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	63,743		67,936
Impaired loans with related allowance	3,686		4,238
Recorded Investment
Impaired loans with no related allowance	46,185		49,731
Impaired loans with related allowance	3,026		2,985
Related Allowance	829		553
Average Recorded Investment
Impaired loans with no related allowance	46,680	65,461
Impaired loans with related allowance	3,032	1,756
Interest Income Recognized
Impaired loans with no related allowance	577	871
Impaired loans with related allowance
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	7,664		8,039
Impaired loans with related allowance	65		65
Recorded Investment
Impaired loans with no related allowance	4,335		4,664
Impaired loans with related allowance	34		34
Related Allowance	23		23
Average Recorded Investment
Impaired loans with no related allowance	4,471	7,746
Impaired loans with related allowance	34	879
Interest Income Recognized
Impaired loans with no related allowance	41	58
Impaired loans with related allowance
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	82		82
Recorded Investment
Impaired loans with no related allowance	43		44
Average Recorded Investment
Impaired loans with no related allowance	43	225
Interest Income Recognized
Impaired loans with no related allowance
Other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance	8		11
Recorded Investment
Impaired loans with no related allowance