Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 7,049,257	$ 6,703,256
Non-Accrual	20,421	28,724
Total Past Due & Non-Accrual	38,814	47,943
Total	7,088,071	6,751,199
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,649,538	1,487,221
Non-Accrual	2,287	3,275
Total Past Due & Non-Accrual	6,031	6,272
Total	1,655,569	1,493,493
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	87,864	120,720
Non-Accrual	640	1,027
Total Past Due & Non-Accrual	640	1,037
Total	88,504	121,757
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	667,142	610,896
Non-Accrual	764	65
Total Past Due & Non-Accrual	1,466	1,323
Total	668,608	612,219
Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,683,494	2,542,048
Non-Accrual	10,406	12,951
Total Past Due & Non-Accrual	16,135	20,643
Total	2,699,629	2,562,691
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	955,383	948,947
Non-Accrual	5,140	9,444
Total Past Due & Non-Accrual	10,510	13,818
Total	965,893	962,765
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	513,936	510,362
Non-Accrual	1,126	1,928
Total Past Due & Non-Accrual	3,367	3,659
Total	517,303	514,021
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	98,044	85,744
Non-Accrual	58	34
Total Past Due & Non-Accrual	665	1,191
Total	98,709	86,935
Finance Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,830	2,527
Total	1,830	2,527
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	392,026	394,791
Total Past Due & Non-Accrual
Total	392,026	394,791
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11,976	16,773
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,344	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		10
30-59 Days Past Due | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		1,193
30-59 Days Past Due | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,859	6,923
30-59 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,873	4,010
30-59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,506	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	394	298
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	6,367	1,522
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	400	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 | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	702
60-89 Days Past Due | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,870	166
60-89 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	497	308
60-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	735	184
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	163	834
60-89 Days Past Due | Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	50	924
90 Days or More Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
90 Days or More Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
90 Days or More Past Due | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		65
90 Days or More Past Due | Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		603
90 Days or More Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		56
90 Days or More Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due		175
90 Days or More Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 50	$ 25