Loans and Allowance - Composition of Impaired Loans by Loan Class (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Unpaid Principal Balance
Impaired loans with no related allowance	$ 116,135		$ 116,135		$ 134,581
Impaired loans with related allowance	4,100		4,100		7,953
Total Impaired Loans	120,235		120,235		142,534
Recorded Investment
Impaired loans with no related allowance	80,666		80,666		90,975
Impaired loans with related allowance	3,492		3,492		6,552
Total Impaired Loans	84,158		84,158		97,527
Related Allowance	1,336		1,336		1,842
Average Recorded Investment
Impaired loans with no related allowance	81,215	$ 88,801	83,884	$ 91,561
Impaired loans with related allowance	3,517	6,362	3,640	6,560
Total Impaired Loans	84,732	95,163	87,524	98,121
Interest Income Recognized
Impaired loans with no related allowance	1,058	1,157	3,173	3,329
Impaired loans with related allowance	6	10	6	29
Total Impaired Loans	1,064	1,167	3,179	3,358
Commercial and industrial loans
Unpaid Principal Balance
Impaired loans with no related allowance	18,096		18,096		22,151
Impaired loans with related allowance	779		779		3,043
Recorded Investment
Impaired loans with no related allowance	9,996		9,996		11,669
Impaired loans with related allowance	467		467		2,690
Related Allowance	379		379		1,247
Average Recorded Investment
Impaired loans with no related allowance	10,025	11,863	10,436	12,676
Impaired loans with related allowance	469	2,731	471	2,774
Interest Income Recognized
Impaired loans with no related allowance	89	137	261	368
Impaired loans with related allowance		10		29
Agricultural production financing and other loans to farmers
Unpaid Principal Balance
Impaired loans with no related allowance	649		649		370
Impaired loans with related allowance	1,186		1,186		466
Recorded Investment
Impaired loans with no related allowance	611		611		361
Impaired loans with related allowance	1,156		1,156		466
Related Allowance	175		175		30
Average Recorded Investment
Impaired loans with no related allowance	636	675	644	699
Impaired loans with related allowance	1,164	547	1,236	547
Interest Income Recognized
Impaired loans with no related allowance	3		3
Impaired loans with related allowance	6		6
Real estate loans - Construction
Unpaid Principal Balance
Impaired loans with no related allowance	6,492		6,492		4,551
Recorded Investment
Impaired loans with no related allowance	3,545		3,545		2,336
Average Recorded Investment
Impaired loans with no related allowance	3,526	2,855	3,572	3,407
Interest Income Recognized
Impaired loans with no related allowance	72	41	215	123
Real estate loans - Commercial and farmland
Unpaid Principal Balance
Impaired loans with no related allowance	82,551		82,551		95,930
Impaired loans with related allowance	2,070		2,070		2,144
Recorded Investment
Impaired loans with no related allowance	61,536		61,536		69,024
Impaired loans with related allowance	1,835		1,835		1,933
Related Allowance	760		760		392
Average Recorded Investment
Impaired loans with no related allowance	61,903	64,186	63,722	65,310
Impaired loans with related allowance	1,850	2,459	1,894	2,613
Interest Income Recognized
Impaired loans with no related allowance	854	932	2,576	2,661
Impaired loans with related allowance
Real estate loans - Residential
Unpaid Principal Balance
Impaired loans with no related allowance	8,252		8,252		11,262
Impaired loans with related allowance	65		65		2,300
Recorded Investment
Impaired loans with no related allowance	4,934		4,934		7,338
Impaired loans with related allowance	34		34		1,463
Related Allowance	22		22		173
Average Recorded Investment
Impaired loans with no related allowance	5,081	9,028	5,453	9,272
Impaired loans with related allowance	34	625	39	626
Interest Income Recognized
Impaired loans with no related allowance	40	47	118	177
Impaired loans with related allowance
Real estate loans - Home equity
Unpaid Principal Balance
Impaired loans with no related allowance	82		82		297
Recorded Investment
Impaired loans with no related allowance	44		44		247
Average Recorded Investment
Impaired loans with no related allowance	44	194	57	$ 197
Interest Income Recognized
Impaired loans with no related allowance
Other commercial loans
Unpaid Principal Balance
Impaired loans with no related allowance	13		13		20
Recorded Investment
Impaired loans with no related allowance