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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 49,499	$ 46,192	$ 48,156	$ 45,779
Provision for loan losses	1,105	1,725	2,415	3,675
Recoveries	296	667	800	1,157
Charge-offs	(949)	(1,174)	(1,420)	(3,201)
Ending balance	49,951	47,410	49,951	47,410
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	9,277	11,682	10,865	11,707
Provision for loan losses	335	(76)	(1,292)	927
Recoveries	126	522	284	749
Charge-offs	(520)	(193)	(639)	(1,448)
Ending balance	9,218	11,935	9,218	11,935
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,512	946	1,305	1,101
Provision for loan losses	(115)	207	88	54
Recoveries	5	2	9	8
Charge-offs		(28)		(36)
Ending balance	1,402	1,127	1,402	1,127
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	32,539	27,279	29,896	26,864
Provision for loan losses	799	1,359	3,233	2,132
Recoveries	18	39	260	91
Charge-offs	(113)	(654)	(146)	(1,064)
Ending balance	33,243	28,023	33,243	28,023
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,171	6,285	6,090	6,107
Provision for loan losses	86	235	386	562
Recoveries	147	104	247	309
Charge-offs	(316)	(299)	(635)	(653)
Ending balance	$ 6,088	$ 6,325	$ 6,088	$ 6,325