Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Unpaid Principal Balance
Impaired loans with no related allowance	$ 72,882		$ 72,882		$ 100,416
Impaired loans with related allowance	4,793		4,793		4,963
Total Impaired Loans	77,675		77,675		105,379
Recorded Investment
Impaired loans with no related allowance	49,783		49,783		68,371
Impaired loans with related allowance	3,861		3,861		3,679
Total Impaired Loans	53,644		53,644		72,050
Related Allowance	881		881		612
Average Recorded Investment
Impaired loans with no related allowance	54,502	$ 82,671	56,365	$ 83,928
Impaired loans with related allowance	3,870	6,267	3,881	6,402
Total Impaired Loans	58,372	88,938	60,246	90,330
Interest Income Recognized
Impaired loans with no related allowance	489	1,119	1,020	2,193
Impaired loans with related allowance		9	2	19
Total Impaired Loans	489	1,128	1,022	2,212
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	6,635		6,635		17,645
Recorded Investment
Impaired loans with no related allowance	3,332		3,332		10,074
Average Recorded Investment
Impaired loans with no related allowance	3,398	10,372	3,629	10,307
Impaired loans with related allowance		2,120		2,129
Interest Income Recognized
Impaired loans with no related allowance	23	128	45	231
Impaired loans with related allowance		9		19
Agricultural production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	672		672		757
Impaired loans with related allowance					660
Recorded Investment
Impaired loans with no related allowance	662		662		680
Impaired loans with related allowance					660
Related Allowance					36
Average Recorded Investment
Impaired loans with no related allowance	663	834	664	882
Impaired loans with related allowance		1,321		1,405
Interest Income Recognized
Impaired loans with no related allowance		2		2
Impaired loans with related allowance
Real estate loans - Construction
Unpaid Principal Balance
Impaired loans with no related allowance	1,238		1,238		5,946
Recorded Investment
Impaired loans with no related allowance	557		557		3,178
Average Recorded Investment
Impaired loans with no related allowance	555	4,085	556	4,074
Interest Income Recognized
Impaired loans with no related allowance	5	74	5	147
Real estate loans - Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	58,749		58,749		67,936
Impaired loans with related allowance	3,988		3,988		4,238
Recorded Investment
Impaired loans with no related allowance	41,750		41,750		49,731
Impaired loans with related allowance	3,276		3,276		2,985
Related Allowance	766		766		553
Average Recorded Investment
Impaired loans with no related allowance	46,343	62,173	47,875	63,136
Impaired loans with related allowance	3,282	1,986	3,289	2,008
Interest Income Recognized
Impaired loans with no related allowance	427	861	903	1,706
Impaired loans with related allowance			2
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	5,575		5,575		8,039
Impaired loans with related allowance	765		765		65
Recorded Investment
Impaired loans with no related allowance	3,473		3,473		4,664
Impaired loans with related allowance	567		567		34
Related Allowance	104		104		23
Average Recorded Investment
Impaired loans with no related allowance	3,534	5,069	3,632	5,390
Impaired loans with related allowance	570	840	574	860
Interest Income Recognized
Impaired loans with no related allowance	34	54	67	107
Impaired loans with related allowance
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	13		13		82
Recorded Investment
Impaired loans with no related allowance	9		9		44
Average Recorded Investment
Impaired loans with no related allowance	9	138	9	139
Impaired loans with related allowance	18		18
Interest Income Recognized
Impaired loans with no related allowance
Impaired loans with related allowance
Other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance					11
Recorded Investment
Impaired loans with no related allowance
Home equity
Unpaid Principal Balance
Impaired loans with related allowance	40		40
Recorded Investment
Impaired loans with related allowance	18		18
Related Allowance	$ 11		$ 11