Loans Held for Investment and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		$ 22,808	$ 22,926	$ 25,523
Total Current		4,018,144	3,930,710	3,805,957
Total Loans		4,040,952	3,953,636	3,831,480
Total 90 Days Past Due Still Accruing		300	1,008	200
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		3,633	4,905	9,064
Total Current		810,254	840,048	765,946
Total Loans		813,887	844,953	775,010
Total 90 Days Past Due Still Accruing		58		74
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		508	836	785
Total Current		97,027	95,841	91,798
Total Loans		97,535	96,677	92,583
Total 90 Days Past Due Still Accruing		30
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		17,707	16,086	14,447
Total Current		2,712,878	2,623,260	2,553,431
Total Loans		2,730,585	2,639,346	2,567,878
Total 90 Days Past Due Still Accruing		180	960	70
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		960	1,099	1,227
Total Current		397,985	371,561	394,782
Total Loans		398,945	372,660	396,009
Total 90 Days Past Due Still Accruing		32	48	56
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	[1]	19,937	18,304	19,936
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	[1]	2,671	3,546	4,950
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	[1]	315	791	626
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	[1]	16,130	13,185	13,441
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	[1]	821	782	919
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,326	1,845	1,950
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		352	682	1,085
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		163	19	159
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		712	881	474
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		99	263	232
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,545	2,777	3,637
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		610	677	3,029
Greater than 90 Days [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		30	26
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		865	2,020	532
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		$ 40	$ 54	$ 76

[1] The Company monitors commercial, agricultural and real estate loans after such loans are 15 days past due. Consumer loans are monitored after such loans are 30 days past due.