Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 6,851,488	$ 6,703,256
Non-Accrual	27,497	28,724
Total Past Due & Non-Accrual	50,208	47,943
Total	6,901,696	6,751,199
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,549,330	1,487,221
Non-Accrual	3,238	3,275
Total Past Due & Non-Accrual	4,839	6,272
Total	1,554,169	1,493,493
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	89,198	120,720
Non-Accrual	638	1,027
Total Past Due & Non-Accrual	9,117	1,037
Total	98,315	121,757
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	588,424	610,896
Non-Accrual	1,481	65
Total Past Due & Non-Accrual	1,669	1,323
Total	590,093	612,219
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,693,020	2,542,048
Non-Accrual	13,944	12,951
Total Past Due & Non-Accrual	20,974	20,643
Total	2,713,994	2,562,691
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	938,960	948,947
Non-Accrual	6,765	9,444
Total Past Due & Non-Accrual	9,684	13,818
Total	948,644	962,765
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	506,922	510,362
Non-Accrual	1,385	1,928
Total Past Due & Non-Accrual	3,623	3,659
Total	510,545	514,021
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	87,933	85,744
Non-Accrual	46	34
Total Past Due & Non-Accrual	302	1,191
Total	88,235	86,935
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,193	2,527
Total	2,193	2,527
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	395,508	394,791
Total Past Due & Non-Accrual
Total	395,508	394,791
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	19,703	16,773
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,062	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	8,479
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	188	1,193
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,589	6,923
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,577	4,010
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,638	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	170	298
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,270	1,522
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	49	30
60-89 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
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	1,378	166
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	305	308
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	481	184
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	57	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	738	924
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	490
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	63	603
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	37	56
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	119	175
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 29	$ 25