Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 7,045,685	$ 6,703,256
Non-Accrual	20,143	28,724
Total Past Due & Non-Accrual	35,374	47,943
Total	7,081,059	6,751,199
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,654,674	1,487,221
Non-Accrual	2,736	3,275
Total Past Due & Non-Accrual	2,917	6,272
Total	1,657,591	1,493,493
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	87,937	120,720
Non-Accrual	640	1,027
Total Past Due & Non-Accrual	1,156	1,037
Total	89,093	121,757
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	713,942	610,896
Non-Accrual	695	65
Total Past Due & Non-Accrual	924	1,323
Total	714,866	612,219
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,635,978	2,542,048
Non-Accrual	9,431	12,951
Total Past Due & Non-Accrual	16,804	20,643
Total	2,652,782	2,562,691
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	956,273	948,947
Non-Accrual	5,522	9,444
Total Past Due & Non-Accrual	9,447	13,818
Total	965,720	962,765
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	515,026	510,362
Non-Accrual	1,082	1,928
Total Past Due & Non-Accrual	3,673	3,659
Total	518,699	514,021
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	92,356	85,744
Non-Accrual	37	34
Total Past Due & Non-Accrual	453	1,191
Total	92,809	86,935
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,945	2,527
Total	1,945	2,527
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	387,554	394,791
Total Past Due & Non-Accrual
Total	387,554	394,791
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9,050	16,773
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	181	2,967
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	200	10
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	229	1,193
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,965	6,923
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,280	4,010
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,947	1,372
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	248	298
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,997	1,522
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		30
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	316
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	4,408	166
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	552	308
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	569	184
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	152	834
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	184	924
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
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		65
Loans 90 Days And Accruing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		603
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	93	56
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	75	175
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 16	$ 25