Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Current	$ 7,470,923	$ 7,186,609
Non-Accrual	25,635	26,148
Total Past Due & Non-Accrual	40,447	37,858
Total	7,511,370	7,224,467
Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Current	1,874,004	1,723,337
Non-Accrual	2,770	1,803
Total Past Due & Non-Accrual	3,038	3,327
Total	1,877,042	1,726,664
Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Current	80,634	89,440
Non-Accrual	2,785	679
Total Past Due & Non-Accrual	2,818	2,964
Total	83,452	92,404
Construction
Financing Receivable, Past Due [Line Items]
Current	617,519	535,520
Non-Accrual	6,918	8,667
Total Past Due & Non-Accrual	7,029	10,209
Total	624,548	545,729
Commercial and farmland
Financing Receivable, Past Due [Line Items]
Current	2,808,422	2,822,515
Non-Accrual	6,340	8,156
Total Past Due & Non-Accrual	13,267	9,587
Total	2,821,689	2,832,102
Residential
Financing Receivable, Past Due [Line Items]
Current	984,086	959,252
Non-Accrual	4,579	4,966
Total Past Due & Non-Accrual	9,716	7,169
Total	993,802	966,421
Home equity
Financing Receivable, Past Due [Line Items]
Current	544,338	524,198
Non-Accrual	1,826	1,481
Total Past Due & Non-Accrual	3,668	3,959
Total	548,006	528,157
Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Current	97,826	99,499
Non-Accrual	64	42
Total Past Due & Non-Accrual	558	289
Total	98,384	99,788
Public finance and other commercial loans
Financing Receivable, Past Due [Line Items]
Current	464,094	432,848
Non-Accrual	353	354
Total Past Due & Non-Accrual	353	354
Total	464,447	433,202
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	13,398	8,794
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	248	1,093
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Past Due	33	2,285
30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Past Due	111	64
30-59 Days Past Due | Commercial and farmland
Financing Receivable, Past Due [Line Items]
Past Due	6,751	1,253
30-59 Days Past Due | Residential
Financing Receivable, Past Due [Line Items]
Past Due	4,540	1,756
30-59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	1,308	2,164
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	407	179
30-59 Days Past Due | Public finance and other commercial loans
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	1,205	1,061
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	20	182
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due | Commercial and farmland
Financing Receivable, Past Due [Line Items]
Past Due	176	178
60-89 Days Past Due | Residential
Financing Receivable, Past Due [Line Items]
Past Due	422	430
60-89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	500	207
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	87	64
60-89 Days Past Due | Public finance and other commercial loans
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Loans 90 Days or More Past Due And Accruing
Financing Receivable, Past Due [Line Items]
Past Due	209	1,855
Loans 90 Days or More Past Due And Accruing | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	0	249
Loans 90 Days or More Past Due And Accruing | Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Loans 90 Days or More Past Due And Accruing | Construction
Financing Receivable, Past Due [Line Items]
Past Due	0	1,478
Loans 90 Days or More Past Due And Accruing | Commercial and farmland
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Loans 90 Days or More Past Due And Accruing | Residential
Financing Receivable, Past Due [Line Items]
Past Due	175	17
Loans 90 Days or More Past Due And Accruing | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	34	107
Loans 90 Days or More Past Due And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	0	4
Loans 90 Days or More Past Due And Accruing | Public finance and other commercial loans
Financing Receivable, Past Due [Line Items]
Past Due	$ 0	$ 0