Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 7,248,608	$ 7,186,609
Non-Accrual	27,949	26,148
Total Past Due & Non-Accrual	50,712	37,858
Total	7,299,320	7,224,467
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,784,771	1,723,337
Non-Accrual	2,498	1,803
Total Past Due & Non-Accrual	3,857	3,327
Total	1,788,628	1,726,664
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	77,435	89,440
Non-Accrual	2,910	679
Total Past Due & Non-Accrual	2,922	2,964
Total	80,357	92,404
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	534,904	535,520
Non-Accrual	7,533	8,667
Total Past Due & Non-Accrual	7,597	10,209
Total	542,501	545,729
Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,818,380	2,822,515
Non-Accrual	8,201	8,156
Total Past Due & Non-Accrual	20,418	9,587
Total	2,838,798	2,832,102
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	965,990	959,252
Non-Accrual	4,473	4,966
Total Past Due & Non-Accrual	10,678	7,169
Total	976,668	966,421
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	531,874	524,198
Non-Accrual	1,963	1,481
Total Past Due & Non-Accrual	4,334	3,959
Total	536,208	528,157
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	107,792	99,499
Non-Accrual	18	42
Total Past Due & Non-Accrual	424	289
Total	108,216	99,788
Public finance and other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	427,462	432,848
Non-Accrual	353	354
Total Past Due & Non-Accrual	482	354
Total	427,944	433,202
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	20,913	8,794
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	977	1,093
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	2,285
30-59 Days Past Due | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	64	64
30-59 Days Past Due | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12,095	1,253
30-59 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,666	1,756
30-59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,646	2,164
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	324	179
30-59 Days Past Due | Public finance and other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	129	0
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,716	1,061
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	382	182
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	178
60-89 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	539	430
60-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	725	207
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	70	64
60-89 Days Past Due | Public finance and other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Loans 90 Days or More Past Due And Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	134	1,855
Loans 90 Days or More Past Due And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, 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, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Loans 90 Days or More Past Due And Accruing | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	1,478
Loans 90 Days or More Past Due And Accruing | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	122	0
Loans 90 Days or More Past Due And Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	17
Loans 90 Days or More Past Due And Accruing | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	107
Loans 90 Days or More Past Due And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12	4
Loans 90 Days or More Past Due And Accruing | Public finance and other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 0	$ 0