Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 5,233,417	$ 5,095,918
Non-Accrual	27,920	29,998
Total Past Due & Non-Accrual	41,492	43,727
Total	5,274,909	5,139,645
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,255,950	1,192,079
Non-Accrual	1,975	1,839
Total Past Due & Non-Accrual	2,890	2,567
Total	1,258,840	1,194,646
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	75,251	78,360
Non-Accrual	665	1,329
Total Past Due & Non-Accrual	1,770	1,329
Total	77,021	79,689
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	336,027	415,975
Non-Accrual	71	73
Total Past Due & Non-Accrual	904	2,728
Total	336,931	418,703
Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,098,174	1,932,896
Non-Accrual	14,536	15,754
Total Past Due & Non-Accrual	20,257	20,166
Total	2,118,431	1,953,062
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	725,168	725,338
Non-Accrual	9,284	9,523
Total Past Due & Non-Accrual	12,750	13,831
Total	737,918	739,169
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	421,202	415,969
Non-Accrual	1,324	1,457
Total Past Due & Non-Accrual	2,506	2,556
Total	423,708	418,525
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	77,175	76,929
Non-Accrual	65	23
Total Past Due & Non-Accrual	415	550
Total	77,590	77,479
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	261	311
Total	261	311
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	244,209	258,061
Total Past Due & Non-Accrual
Total	244,209	258,061
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11,956	9,490
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	915	466
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,105
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	833	2,655
30-59 Days Past Due | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,076	1,385
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,836	3,664
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	906	850
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	285	470
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,493	4,127
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		162
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 | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	645	3,027
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	630	635
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	175	246
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	43	57
60-89 Days Past Due | Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
Loans 90 Days And Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	123	112
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		100
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 | 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		9
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	101	3
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 22