Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 6,439,057	$ 5,095,918
Non-Accrual	32,253	29,998
Total Past Due & Non-Accrual	44,391	43,727
Total	6,483,448	5,139,645
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,430,469	1,192,079
Non-Accrual	4,910	1,839
Total Past Due & Non-Accrual	5,623	2,567
Total	1,436,092	1,194,646
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	116,410	78,360
Non-Accrual	1,282	1,329
Total Past Due & Non-Accrual	1,341	1,329
Total	117,751	79,689
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	498,233	415,975
Non-Accrual	67	73
Total Past Due & Non-Accrual	629	2,728
Total	498,862	418,703
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,555,261	1,932,896
Non-Accrual	12,152	15,754
Total Past Due & Non-Accrual	15,992	20,166
Total	2,571,253	1,953,062
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	922,848	725,338
Non-Accrual	11,973	9,523
Total Past Due & Non-Accrual	15,589	13,831
Total	938,437	739,169
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	497,692	415,969
Non-Accrual	1,818	1,457
Total Past Due & Non-Accrual	4,548	2,556
Total	502,240	418,525
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	85,877	76,929
Non-Accrual	51	23
Total Past Due & Non-Accrual	529	550
Total	86,406	77,479
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,877	311
Total	3,877	311
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	328,390	258,061
Total Past Due & Non-Accrual	140
Total	328,530	258,061
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8,779	9,490
30-59 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	196	466
30-59 Days Past Due | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	59
30-59 Days Past Due | Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	432	2,655
30-59 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,952	1,385
30-59 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,960	3,664
30-59 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,880	850
30-59 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	300	470
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,934	4,127
60-89 Days Past Due | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	492	162
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	62
60-89 Days Past Due | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	879	3,027
60-89 Days Past Due | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	613	635
60-89 Days Past Due | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	585	246
60-89 Days Past Due | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	163	57
60-89 Days Past Due | Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	140
Loans 90 Days And Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	425	112
Loans 90 Days And Accruing | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	25	100
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	68
Loans 90 Days And Accruing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	9
Loans 90 Days And Accruing | Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	43	9
Loans 90 Days And Accruing | Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	265	3
Loans 90 Days And Accruing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 15