Loans Held for Investment and Allowance for Credit Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 52,499	$ 51,202
Provision for loan losses	9,850	965
Recoveries	318	882
Charge-offs	(2,227)	(1,464)
Ending balance	60,440	51,585
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	12,122	11,948
Provision for loan losses	794	196
Recoveries	149,000	649
Charge-offs	(1,292)	(318)
Ending balance	11,773	12,475
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,206	1,446
Provision for loan losses	949	(19)
Recoveries	1	3
Charge-offs	(2)
Ending balance	2,154	1,430
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	33,974	32,342
Provision for loan losses	7,921	397
Recoveries	76	89
Charge-offs	(715,000)	(941)
Ending balance	41,256	31,887
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5,197	5,466
Provision for loan losses	186	391
Recoveries	92	141
Charge-offs	(218)	(205)
Ending balance	$ 5,257	$ 5,793