Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 4,267,367	$ 3,846,157
Non-Accrual	32,597	48,789
Total Past Due & Non-Accrual	54,348	78,708
Total	4,321,715	3,924,865
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	992,529	882,596
Non-Accrual	4,460	7,048
Total Past Due & Non-Accrual	6,666	14,092
Total	999,195	896,688
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	86,997	98,236
Non-Accrual	1,210	5,800
Total Past Due & Non-Accrual	4,357	6,691
Total	91,354	104,927
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	296,978	204,683
Non-Accrual	745	1,439
Total Past Due & Non-Accrual	1,272	2,538
Total	298,250	207,221
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,673,750	1,642,016
Non-Accrual	13,506	19,350
Total Past Due & Non-Accrual	21,953	30,645
Total	1,695,703	1,672,661
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	661,884	626,821
Non-Accrual	10,749	12,933
Total Past Due & Non-Accrual	15,883	20,494
Total	677,767	647,315
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	315,250	282,828
Non-Accrual	1,783	1,988
Total Past Due & Non-Accrual	3,699	3,701
Total	318,949	286,529
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	71,375	72,853
Non-Accrual	144	231
Total Past Due & Non-Accrual	518	547
Total	71,893	73,400
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 614	$ 1,106
Total Past Due & Non-Accrual
Total	$ 614	$ 1,106
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 167,990	$ 35,018
Total Past Due & Non-Accrual
Total	$ 167,990	$ 35,018
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	14,397	22,107
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,106	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	2,647	891
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	504	1,017
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,935	9,846
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,128	4,876
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	793	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	$ 284	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	$ 5,407	3,149
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,035	$ 53
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	500
60-89 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	23	$ 82
60-89 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,426	778
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,531	1,831
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	861	352
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	31	53
Loans 90 Days And Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,947	4,663
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 65	$ 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
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	$ 1,086	$ 671
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	475	854
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	262	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