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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 60,440	$ 51,585	$ 52,499	$ 51,202
Provision for loan losses	8,700	600	18,550	1,565
Recoveries	701	697	1,019	1,578
Charge-offs	(894)	(1,062)	(3,121)	(2,525)
Ending balance	68,947	51,820	68,947	51,820
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	11,773	12,475	12,122	11,948
Provision for loan losses	6,820	1,418	7,615	1,612
Recoveries	540	423	690	1,073
Charge-offs	(561)	(417)	(1,855)	(734)
Ending balance	18,572	13,899	18,572	13,899
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,154	1,430	1,206	1,446
Provision for loan losses	361	61	1,310	42
Recoveries	29	5	30	7
Charge-offs	0	(136)	(2)	(135)
Ending balance	2,544	1,360	2,544	1,360
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	41,256	31,887	33,974	32,342
Provision for loan losses	1,469	(929)	9,390	(531)
Recoveries	59	60	135	149
Charge-offs	(161)	(219)	(876)	(1,161)
Ending balance	42,623	30,799	42,623	30,799
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5,257	5,793	5,197	5,466
Provision for loan losses	50	50	235	442
Recoveries	73	209	164	349
Charge-offs	(172)	(290)	(388)	(495)
Ending balance	$ 5,208	$ 5,762	$ 5,208	$ 5,762