Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Current	$ 8,259,461	$ 7,186,609
Non-Accrual	22,728	26,148
Total Past Due & Non-Accrual	39,799	37,858
Total	8,299,260	7,224,467
Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Current	2,026,547	1,723,337
Non-Accrual	2,065	1,803
Total Past Due & Non-Accrual	7,764	3,327
Total	2,034,311	1,726,664
Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Current	87,274	89,440
Non-Accrual	3,150	679
Total Past Due & Non-Accrual	3,188	2,964
Total	90,462	92,404
Construction
Financing Receivable, Past Due [Line Items]
Current	669,111	535,520
Non-Accrual	1,027	8,667
Total Past Due & Non-Accrual	1,027	10,209
Total	670,138	545,729
Commercial and farmland
Financing Receivable, Past Due [Line Items]
Current	3,132,460	2,822,515
Non-Accrual	9,265	8,156
Total Past Due & Non-Accrual	12,189	9,587
Total	3,144,649	2,832,102
Residential
Financing Receivable, Past Due [Line Items]
Current	1,145,240	959,252
Non-Accrual	5,381	4,966
Total Past Due & Non-Accrual	10,163	7,169
Total	1,155,403	966,421
Home equity
Financing Receivable, Past Due [Line Items]
Current	596,472	524,198
Non-Accrual	1,737	1,481
Total Past Due & Non-Accrual	4,863	3,959
Total	601,335	528,157
Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Current	130,641	99,499
Non-Accrual	103	42
Total Past Due & Non-Accrual	605	289
Total	131,246	99,788
Public finance and other commercial loans
Financing Receivable, Past Due [Line Items]
Current	471,716	432,848
Non-Accrual	0	354
Total Past Due & Non-Accrual	0	354
Total	471,716	433,202
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	14,791	8,794
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	5,468	1,093
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Past Due	0	2,285
30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Past Due	0	64
30-59 Days Past Due | Commercial and farmland
Financing Receivable, Past Due [Line Items]
Past Due	2,219	1,253
30-59 Days Past Due | Residential
Financing Receivable, Past Due [Line Items]
Past Due	4,238	1,756
30-59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	2,547	2,164
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	319	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	2,198	1,061
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	231	182
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Past Due	38	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	705	178
60-89 Days Past Due | Residential
Financing Receivable, Past Due [Line Items]
Past Due	526	430
60-89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	515	207
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	183	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	82	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	18	17
Loans 90 Days or More Past Due And Accruing | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	64	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