Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 4,926,983	$ 4,641,215
Non-Accrual	34,105	31,389
Total Past Due & Non-Accrual	46,861	52,607
Total	4,973,844	4,693,822
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,143,456	1,052,275
Non-Accrual	2,381	4,634
Total Past Due & Non-Accrual	3,082	4,800
Total	1,146,538	1,057,075
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	91,639	96,884
Non-Accrual	1,530	827
Total Past Due & Non-Accrual	1,530	827
Total	93,169	97,711
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	367,969	362,084
Non-Accrual	271	736
Total Past Due & Non-Accrual	272	4,620
Total	368,241	366,704
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,919,826	1,786,092
Non-Accrual	16,983	11,277
Total Past Due & Non-Accrual	21,913	16,829
Total	1,941,739	1,802,921
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	724,200	765,634
Non-Accrual	10,608	11,818
Total Past Due & Non-Accrual	15,655	20,471
Total	739,855	786,105
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	394,977	344,344
Non-Accrual	2,216	1,952
Total Past Due & Non-Accrual	3,860	4,269
Total	398,837	348,613
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	75,948	73,990
Non-Accrual	116	145
Total Past Due & Non-Accrual	549	727
Total	76,497	74,717
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	380	588
Total	380	588
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	208,588	159,324
Total Past Due & Non-Accrual		64
Total	208,588	159,388
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	6,203	17,570
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	701	166
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
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	976	5,552
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,390	6,090
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	841	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	295	445
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,928	2,741
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
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	2,737
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,574	2,061
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	510	560
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	107	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	1,625	907
Loans 90 Days And Accruing | Commercial and industrial loans
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	1
Loans 90 Days And Accruing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,217
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	83	502
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	293	324
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 31	$ 81