Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 4,657,009	$ 4,641,215
Non-Accrual	36,719	31,389
Total Past Due & Non-Accrual	52,898	52,607
Total	4,709,907	4,693,822
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,054,549	1,052,275
Non-Accrual	3,694	4,634
Total Past Due & Non-Accrual	6,010	4,800
Total	1,060,559	1,057,075
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	90,452	96,884
Non-Accrual	1,535	827
Total Past Due & Non-Accrual	1,685	827
Total	92,137	97,711
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	385,222	362,084
Non-Accrual	731	736
Total Past Due & Non-Accrual	6,399	4,620
Total	391,621	366,704
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,788,591	1,786,092
Non-Accrual	16,440	11,277
Total Past Due & Non-Accrual	17,804	16,829
Total	1,806,395	1,802,921
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	753,121	765,634
Non-Accrual	11,674	11,818
Total Past Due & Non-Accrual	16,471	20,471
Total	769,592	786,105
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	349,986	344,344
Non-Accrual	2,449	1,952
Total Past Due & Non-Accrual	3,960	4,269
Total	353,946	348,613
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	73,053	73,990
Non-Accrual	196	145
Total Past Due & Non-Accrual	569	727
Total	73,622	74,717
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 461	$ 588
Total Past Due & Non-Accrual
Total	$ 461	$ 588
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 161,574	159,324
Total Past Due & Non-Accrual		64
Total	$ 161,574	159,388
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	14,221	17,570
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,017	$ 166
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	150
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,668	$ 3,884
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,355	5,552
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,565	6,090
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,298	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	$ 168	445
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	$ 995	$ 2,741
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 239
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
60-89 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
60-89 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 9
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	539	$ 2,061
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	107	560
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	101	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	963	$ 907
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 60
Loans 90 Days And Accruing | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
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
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 693	$ 502
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	106	324
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 104	$ 81