Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 3,904,616	$ 3,846,157
30-59 Days Past Due	12,312	22,107
60-89 Days Past Due	2,628	3,149
Loans 90 Days And Accruing	1,655	4,663
Non-Accrual	44,321	48,789
Total Past Due & Non-Accrual	60,916	78,708
Total	3,965,532	3,924,865
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	927,244	882,596
30-59 Days Past Due	2,057	4,006
60-89 Days Past Due	1,166	53
Loans 90 Days And Accruing	221	2,985
Non-Accrual	8,249	7,048
Total Past Due & Non-Accrual	11,693	14,092
Total	938,937	896,688
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	93,349	98,236
30-59 Days Past Due	325	891
60-89 Days Past Due
Non-Accrual	1,978	5,800
Total Past Due & Non-Accrual	2,303	6,691
Total	95,652	104,927
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	235,077	204,683
30-59 Days Past Due	138	1,017
60-89 Days Past Due		82
Loans 90 Days And Accruing	236
Non-Accrual	1,585	1,439
Total Past Due & Non-Accrual	1,959	2,538
Total	237,036	207,221
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,623,656	1,642,016
30-59 Days Past Due	5,261	9,846
60-89 Days Past Due	305	778
Loans 90 Days And Accruing	265	671
Non-Accrual	16,931	19,350
Total Past Due & Non-Accrual	22,762	30,645
Total	1,646,418	1,672,661
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	623,394	626,821
30-59 Days Past Due	2,446	4,876
60-89 Days Past Due	761	1,831
Loans 90 Days And Accruing	536	854
Non-Accrual	13,314	12,933
Total Past Due & Non-Accrual	17,057	20,494
Total	640,451	647,315
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	282,344	282,828
30-59 Days Past Due	1,734	1,213
60-89 Days Past Due	316	352
Loans 90 Days And Accruing	379	148
Non-Accrual	2,141	1,988
Total Past Due & Non-Accrual	4,570	3,701
Total	286,914	286,529
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	69,829	72,853
30-59 Days Past Due	173	258
60-89 Days Past Due	80	53
Loans 90 Days And Accruing	18	5
Non-Accrual	123	231
Total Past Due & Non-Accrual	394	547
Total	70,223	73,400
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	675	1,106
30-59 Days Past Due	178
Total Past Due & Non-Accrual	178
Total	853	1,106
Other loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	49,048	35,018
30-59 Days Past Due
Non-Accrual
Total Past Due & Non-Accrual
Total	$ 49,048	$ 35,018