Loans Held for Investment and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 51,585	$ 49,499	$ 51,202	$ 48,156
Provision for loan losses	600	1,105	1,565	2,415
Recoveries	697	296	1,578	800
Charge-offs	(1,062)	(949)	(2,525)	(1,420)
Ending balance	51,820	49,951	51,820	49,951
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	12,475	9,277	11,948	10,865
Provision for loan losses	1,418	335	1,612	(1,292)
Recoveries	423	126	1,073	284
Charge-offs	(417)	(520)	(734)	(639)
Ending balance	13,899	9,218	13,899	9,218
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,430	1,512	1,446	1,305
Provision for loan losses	61	(115)	42	88
Recoveries	5	5	7	9
Charge-offs	(136)		(135)
Ending balance	1,360	1,402	1,360	1,402
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	31,887	32,539	32,342	29,896
Provision for loan losses	(929)	799	(531)	3,233
Recoveries	60	18	149	260
Charge-offs	(219)	(113)	(1,161)	(146)
Ending balance	30,799	33,243	30,799	33,243
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5,793	6,171	5,466	6,090
Provision for loan losses	50	86	442	386
Recoveries	209	147	349	247
Charge-offs	(290)	(316)	(495)	(635)
Ending balance	$ 5,762	$ 6,088	$ 5,762	$ 6,088