Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 6,703,256	$ 5,095,918
Loans 90 Days And Accruing	924	112
Non-Accrual	28,724	29,998
Total Past Due & Non-Accrual	47,943	43,727
Total	6,751,199	5,139,645
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	16,773	9,490
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,522	4,127
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,487,221	1,192,079
Loans 90 Days And Accruing		100
Non-Accrual	3,275	1,839
Total Past Due & Non-Accrual	6,272	2,567
Total	1,493,493	1,194,646
Commercial and industrial loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,967	466
Commercial and industrial loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	30	162
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	120,720	78,360
Loans 90 Days And Accruing
Non-Accrual	1,027	1,329
Total Past Due & Non-Accrual	1,037	1,329
Total	121,757	79,689
Agricultural production financing and other loans to farmers | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10
Agricultural production financing and other loans to farmers | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	610,896	415,975
Loans 90 Days And Accruing	65
Non-Accrual	65	73
Total Past Due & Non-Accrual	1,323	2,728
Total	612,219	418,703
Construction | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,193	2,655
Construction | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due
Commercial and farmland
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,542,048	1,932,896
Loans 90 Days And Accruing	603
Non-Accrual	12,951	15,754
Total Past Due & Non-Accrual	20,643	20,166
Total	2,562,691	1,953,062
Commercial and farmland | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	6,923	1,385
Commercial and farmland | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	166	3,027
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	948,947	725,338
Loans 90 Days And Accruing	56	9
Non-Accrual	9,444	9,523
Total Past Due & Non-Accrual	13,818	13,831
Total	962,765	739,169
Residential | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,010	3,664
Residential | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	308	635
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	510,362	415,969
Loans 90 Days And Accruing	175	3
Non-Accrual	1,928	1,457
Total Past Due & Non-Accrual	3,659	2,556
Total	514,021	418,525
Home equity | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,372	850
Home equity | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	184	246
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	85,744	76,929
Loans 90 Days And Accruing	25
Non-Accrual	34	23
Total Past Due & Non-Accrual	1,191	550
Total	86,935	77,479
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	298	470
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	834	57
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,527	311
Non-Accrual
Total Past Due & Non-Accrual
Total	2,527	311
Other commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	394,791	258,061
Non-Accrual
Total Past Due & Non-Accrual
Total	394,791	258,061
Other commercial loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due