Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Unpaid Principal Balance
Impaired loans with no related allowance	$ 131,555		$ 134,581
Impaired loans with related allowance	5,613		7,953
Total Impaired Loans	137,168		142,534
Recorded Investment
Impaired loans with no related allowance	88,827		90,975
Impaired loans with related allowance	5,122		6,552
Total Impaired Loans	93,949		97,527
Related Allowance	1,373		1,842
Average Recorded Investment
Impaired loans with no related allowance	90,462	$ 103,092
Impaired loans with related allowance	5,313	13,244
Total Impaired Loans	95,775	116,336
Interest Income Recognized
Impaired loans with no related allowance	1,124	1,172
Impaired loans with related allowance	9	10
Total Impaired Loans	1,133	1,182
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	21,751		22,151
Impaired loans with related allowance	1,610		3,043
Recorded Investment
Impaired loans with no related allowance	11,559		11,669
Impaired loans with related allowance	1,357		2,690
Related Allowance	620		1,247
Average Recorded Investment
Impaired loans with no related allowance	12,052	16,150
Impaired loans with related allowance	1,363	4,379
Interest Income Recognized
Impaired loans with no related allowance	109	139
Impaired loans with related allowance	9	10
Agricultural production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	724		370
Impaired loans with related allowance	1,150		466
Recorded Investment
Impaired loans with no related allowance	716		361
Impaired loans with related allowance	1,150		466
Related Allowance	415		30
Average Recorded Investment
Impaired loans with no related allowance	716	303
Impaired loans with related allowance	1,315	$ 3,991
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	$ 8,482		4,551
Recorded Investment
Impaired loans with no related allowance	4,300		2,336
Average Recorded Investment
Impaired loans with no related allowance	4,262	$ 8,542
Interest Income Recognized
Impaired loans with no related allowance	85	104
Real estate loans - Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	88,293		95,930
Impaired loans with related allowance	1,907		2,144
Recorded Investment
Impaired loans with no related allowance	64,710		69,024
Impaired loans with related allowance	1,738		1,933
Related Allowance	242		392
Average Recorded Investment
Impaired loans with no related allowance	65,461	69,036
Impaired loans with related allowance	1,756	3,113
Interest Income Recognized
Impaired loans with no related allowance	$ 871	$ 878
Impaired loans with related allowance
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	$ 11,991		11,262
Impaired loans with related allowance	946		2,300
Recorded Investment
Impaired loans with no related allowance	7,317		7,338
Impaired loans with related allowance	877		1,463
Related Allowance	96		173
Average Recorded Investment
Impaired loans with no related allowance	7,746	$ 8,528
Impaired loans with related allowance	879	1,761
Interest Income Recognized
Impaired loans with no related allowance	$ 58	$ 48
Impaired loans with related allowance
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	$ 296		297
Recorded Investment
Impaired loans with no related allowance	225		247
Average Recorded Investment
Impaired loans with no related allowance	$ 225	$ 533
Interest Income Recognized
Impaired loans with no related allowance		$ 3
Other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance	$ 18		$ 20
Recorded Investment
Impaired loans with no related allowance