Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 23,447	$ 29,511	$ 33,312
Total Current	3,448,780	3,327,796	3,304,481
Total Loans	3,472,227	3,357,307	3,337,793
Total 90 Days Past Due Still Accruing	257	284	107
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,368	7,250	12,498
Total Current	669,579	667,160	651,083
Total Loans	674,947	674,410	663,581
Total 90 Days Past Due Still Accruing	212	10	61
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	322	185	586
Total Current	82,683	83,836	84,130
Total Loans	83,005	84,021	84,716
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15,913	19,741	18,316
Total Current	2,281,643	2,170,103	2,172,944
Total Loans	2,297,556	2,189,844	2,191,260
Total 90 Days Past Due Still Accruing		272	34
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,844	2,335	1,912
Total Current	414,875	406,697	396,324
Total Loans	416,719	409,032	398,236
Total 90 Days Past Due Still Accruing	45	2	12
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	17,457	19,110	20,092
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,288	3,908	4,707
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	322	185	523
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,636	13,172	13,444
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,211	1,845	1,418
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,026	2,791	2,714
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	585	1,122	841
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due			63
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	984	1,301	1,496
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	457	368	314
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,964	7,610	10,506
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,495	2,220	6,950
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,293	5,268	3,376
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 176	$ 122	$ 180