Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 30,764	$ 29,511	$ 24,782
Total Current	3,341,748	3,327,796	3,257,417
Total Loans	3,372,512	3,357,307	3,282,199
Total 90 Days Past Due Still Accruing	190	284	59
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,278	7,250	6,064
Total Current	658,409	667,160	663,461
Total Loans	666,687	674,410	669,525
Total 90 Days Past Due Still Accruing		10	42
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	684	185	320
Total Current	78,553	83,836	87,170
Total Loans	79,237	84,021	87,490
Total 90 Days Past Due Still Accruing	15
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	20,389	19,741	16,223
Total Current	2,188,632	2,170,103	2,133,909
Total Loans	2,209,021	2,189,844	2,150,132
Total 90 Days Past Due Still Accruing	151	272
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,413	2,335	2,175
Total Current	416,154	406,697	372,877
Total Loans	417,567	409,032	375,052
Total 90 Days Past Due Still Accruing	24	2	17
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	20,570	19,110	16,314
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,452	3,908	3,338
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	626	185	320
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13,434	13,172	10,979
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,058	1,845	1,677
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,545	2,791	3,124
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	376	1,122	1,228
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	31
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,870	1,301	1,560
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	268	368	336
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7,649	7,610	5,344
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,450	2,220	1,498
Greater than 90 Days [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	27
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,085	5,268	3,684
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 87	$ 122	$ 162