Loans Held for Investment and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 48,156	$ 45,779
Provision for loan losses	5,665	6,530
Recoveries	1,828	1,668
Charge-offs	(4,447)	(5,821)
Ending balance	51,202	48,156
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	10,865	11,707
Provision for loan losses	1,662	1,233
Recoveries	839	943
Charge-offs	(1,418)	(3,018)
Ending balance	11,948	10,865
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,305	1,101
Provision for loan losses	126	243
Recoveries	15	32
Charge-offs		(71)
Ending balance	1,446	1,305
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	29,896	26,864
Provision for loan losses	3,463	4,055
Recoveries	462	192
Charge-offs	(1,479)	(1,215)
Ending balance	32,342	29,896
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,090	6,107
Provision for loan losses	414	999
Recoveries	512	501
Charge-offs	(1,550)	(1,517)
Ending balance	$ 5,466	$ 6,090