Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 4,179,188	$ 3,846,157
Non-Accrual	37,713	48,789
Total Past Due & Non-Accrual	59,017	78,708
Total	4,238,205	3,924,865
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	971,571	882,596
Non-Accrual	5,510	7,048
Total Past Due & Non-Accrual	12,652	14,092
Total	984,223	896,688
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	92,302	98,236
Non-Accrual	1,298	5,800
Total Past Due & Non-Accrual	1,393	6,691
Total	93,695	104,927
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	253,330	204,683
Non-Accrual	1,493	1,439
Total Past Due & Non-Accrual	2,752	2,538
Total	256,082	207,221
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,685,194	1,642,016
Non-Accrual	16,964	19,350
Total Past Due & Non-Accrual	20,453	30,645
Total	1,705,647	1,672,661
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	672,593	626,821
Non-Accrual	10,515	12,933
Total Past Due & Non-Accrual	17,028	20,494
Total	689,621	647,315
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	298,161	282,828
Non-Accrual	1,874	1,988
Total Past Due & Non-Accrual	4,242	3,701
Total	302,403	286,529
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	62,288	72,853
Non-Accrual	59	231
Total Past Due & Non-Accrual	497	547
Total	62,785	73,400
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 742	$ 1,106
Total Past Due & Non-Accrual
Total	$ 742	$ 1,106
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 143,007	$ 35,018
Total Past Due & Non-Accrual
Total	$ 143,007	$ 35,018
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13,308	22,107
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,679	4,006
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	34	891
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,069	1,017
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,090	9,846
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,470	4,876
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,666	1,213
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 300	258
30-59 Days Past Due | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
30-59 Days Past Due | Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 7,364	3,149
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,463	$ 53
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	50
60-89 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	190	$ 82
60-89 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	17	778
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	937	1,831
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	628	352
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	79	53
Loans 90 Days And Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 632	4,663
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		$ 2,985
Loans 90 Days And Accruing | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 11
Loans 90 Days And Accruing | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
Loans 90 Days And Accruing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 382	$ 671
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	106	854
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	74	148
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 59	$ 5