Loans Held for Investment and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 62,138,000	$ 68,947,000	$ 66,534,000	$ 52,499,000
Provision for loan losses	0	7,500,000	(4,468,000)	26,050,000
Recoveries	2,707,000	444,000	3,745,000	1,464,000
Charge-offs	(1,475,000)	(853,000)	(2,441,000)	(3,975,000)
Ending balance	63,370,000	76,038,000	63,370,000	76,038,000
C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	10,186,000		13,609,000
Provision for loan losses	(31,000)		(3,597,000)
Recoveries	1,140,000		1,671,000
Charge-offs	(902,000)		(1,290,000)
Ending balance	10,393,000		10,393,000
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	588,000		1,552,000
Provision for loan losses	(273,000)		(1,237,000)
Ending balance	315,000		315,000
Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	15,632,000		13,512,000
Provision for loan losses	1,230,000		3,348,000
Recoveries			2,000
Ending balance	16,862,000		16,862,000
Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	607,000		1,876,000
Provision for loan losses	(21,000)		(1,300,000)
Recoveries			10,000
Ending balance	586,000		586,000
Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	10,432,000		8,391,000
Provision for loan losses	(344,000)		1,609,000
Recoveries	556,000		650,000
Charge-offs			(6,000)
Ending balance	10,644,000		10,644,000
Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	10,399,000		12,347,000
Provision for loan losses	(94,000)		(2,044,000)
Recoveries	806,000		816,000
Charge-offs	(222,000)		(230,000)
Ending balance	10,889,000		10,889,000
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	11,234,000		12,601,000
Provision for loan losses	(1,184,000)		(2,542,000)
Recoveries	7,000		90,000
Charge-offs			(92,000)
Ending balance	10,057,000		10,057,000
Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,266,000		1,020,000
Provision for loan losses	(259,000)		139,000
Recoveries	129,000		305,000
Charge-offs	(176,000)		504,000
Ending balance	960,000		960,000
Non-Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	482,000		371,000
Provision for loan losses	(46,000)		80,000
Recoveries	64,000		172,000
Charge-offs	(125,000)		(248,000)
Ending balance	375,000		375,000
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance		18,572,000		12,122,000
Provision for loan losses		1,955,000		9,571,000
Recoveries		200,000		890,000
Charge-offs		(536,000)		(2,392,000)
Ending balance		20,191,000		20,191,000
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,312,000	2,544,000	1,255,000	1,206,000
Provision for loan losses	1,022,000	214,000	1,076,000	1,096,000
Recoveries	5,000	1,000	29,000	31,000
Charge-offs	(50,000)	0	(71,000)	(2,000)
Ending balance	$ 2,289,000	2,331,000	$ 2,289,000	2,331,000
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance		42,623,000		33,974,000
Provision for loan losses		4,416,000		13,806,000
Recoveries		138,000		272,000
Charge-offs		(52,000)		(927,000)
Ending balance		47,125,000		47,125,000
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance		5,208,000		5,197,000
Provision for loan losses		1,343,000		1,577,000
Recoveries		105,000		271,000
Charge-offs		(265,000)		(654,000)
Ending balance		$ 6,391,000		$ 6,391,000