Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 9,156,460	$ 8,427,414
Non-Accrual	56,739	15,949
Total Past Due & Non-Accrual	87,373	31,896
Total	9,243,833	8,459,310
Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Current	2,864,852	2,105,445
Non-Accrual	5,947	1,255
Total Past Due & Non-Accrual	10,479	4,434
Total	2,875,331	2,109,879
Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Current	81,990	93,678
Non-Accrual	0	183
Total Past Due & Non-Accrual	1,100	183
Total	83,090	93,861
Construction
Financing Receivable, Past Due [Line Items]
Current	614,383	784,961
Non-Accrual	126	977
Total Past Due & Non-Accrual	7,701	2,607
Total	622,084	787,568
Commercial and farmland
Financing Receivable, Past Due [Line Items]
Current	3,194,394	3,043,318
Non-Accrual	43,030	7,007
Total Past Due & Non-Accrual	54,112	9,380
Total	3,248,506	3,052,698
Residential
Financing Receivable, Past Due [Line Items]
Current	1,137,688	1,133,476
Non-Accrual	4,914	5,062
Total Past Due & Non-Accrual	8,718	9,741
Total	1,146,406	1,143,217
Home equity
Financing Receivable, Past Due [Line Items]
Current	522,680	584,023
Non-Accrual	2,674	1,421
Total Past Due & Non-Accrual	4,778	4,961
Total	527,458	588,984
Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Current	124,926	135,399
Non-Accrual	48	44
Total Past Due & Non-Accrual	485	590
Total	125,411	135,989
Public finance and other commercial loans
Financing Receivable, Past Due [Line Items]
Current	615,547	547,114
Non-Accrual	0	0
Total Past Due & Non-Accrual	0
Total	615,547	547,114
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	17,527	14,683
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	4,529	3,039
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Past Due [Line Items]
Past Due	0	0
30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Past Due	7,575	1,630
30-59 Days Past Due | Commercial and farmland
Financing Receivable, Past Due [Line Items]
Past Due	964	2,324
30-59 Days Past Due | Residential
Financing Receivable, Past Due [Line Items]
Past Due	2,743	4,290
30-59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	1,467	2,960
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	249	440
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	11,777	1,195
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	0	136
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	9,978	49
60-89 Days Past Due | Residential
Financing Receivable, Past Due [Line Items]
Past Due	1,000	367
60-89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	611	538
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Past Due [Line Items]
Past Due	188	105
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	1,330	69
Loans 90 Days or More Past Due And Accruing | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Past Due	3	4
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	1,100	0
Loans 90 Days or More Past Due And Accruing | Construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Loans 90 Days or More Past Due And Accruing | Commercial and farmland
Financing Receivable, Past Due [Line Items]
Past Due	140	0
Loans 90 Days or More Past Due And Accruing | Residential
Financing Receivable, Past Due [Line Items]
Past Due	61	22
Loans 90 Days or More Past Due And Accruing | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	26	42
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	1
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