Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 25,332	$ 20,737	$ 30,764
Total Current	3,704,719	3,464,832	3,341,748
Total Loans	3,730,051	3,485,569	3,372,512
Total 90 Days Past Due Still Accruing	327	288	190
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,465	4,224	8,278
Total Current	727,691	679,875	658,409
Total Loans	734,156	684,099	666,687
Total 90 Days Past Due Still Accruing	204	7
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	332	640	684
Total Current	95,626	93,903	78,553
Total Loans	95,958	94,543	79,237
Total 90 Days Past Due Still Accruing			15
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	17,329	14,017	20,389
Total Current	2,485,575	2,288,981	2,188,632
Total Loans	2,502,904	2,302,998	2,209,021
Total 90 Days Past Due Still Accruing	76	216	151
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,206	1,856	1,413
Total Current	395,827	402,073	416,154
Total Loans	397,033	403,929	417,567
Total 90 Days Past Due Still Accruing	47	65	24
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	21,832	16,347	20,570
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,639	2,039	5,452
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	332	640	626
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	16,037	12,308	13,434
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	824	1,360	1,058
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,525	1,976	2,545
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	760	1,104	376
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	544	511	1,870
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	221	361	268
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,975	2,414	7,649
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,066	1,081	2,450
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	748	1,198	5,085
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 161	$ 135	$ 87