Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 4,744,980	$ 4,641,215
Non-Accrual	33,565	31,389
Total Past Due & Non-Accrual	46,449	52,607
Total	4,791,429	4,693,822
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,080,184	1,052,275
Non-Accrual	3,413	4,634
Total Past Due & Non-Accrual	4,706	4,800
Total	1,084,890	1,057,075
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	92,979	96,884
Non-Accrual	1,471	827
Total Past Due & Non-Accrual	2,152	827
Total	95,131	97,711
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	351,147	362,084
Non-Accrual	721	736
Total Past Due & Non-Accrual	1,833	4,620
Total	352,980	366,704
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,850,882	1,786,092
Non-Accrual	15,541	11,277
Total Past Due & Non-Accrual	18,821	16,829
Total	1,869,703	1,802,921
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	743,951	765,634
Non-Accrual	9,952	11,818
Total Past Due & Non-Accrual	14,919	20,471
Total	758,870	786,105
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	370,602	344,344
Non-Accrual	2,345	1,952
Total Past Due & Non-Accrual	3,557	4,269
Total	374,159	348,613
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	74,744	73,990
Non-Accrual	122	145
Total Past Due & Non-Accrual	461	727
Total	75,205	74,717
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	388	588
Total	388	588
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	180,103	159,324
Total Past Due & Non-Accrual		64
Total	180,103	159,388
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8,404	17,570
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	998	166
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	250
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		3,884
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,384	5,552
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,847	6,090
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	628	1,433
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	297	445
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,118	2,741
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	282
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	431
60-89 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,112
60-89 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	896
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	888	2,061
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	467	560
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	42	56
60-89 Days Past Due | Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		64
Loans 90 Days And Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	362	907
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	232	502
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 117	324
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		$ 81