Loans Held for Investment and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 49,951	$ 47,410	$ 48,156	$ 45,779
Provision for loan losses	1,450	1,415	3,865	5,090
Recoveries	415	277	1,215	1,434
Charge-offs	(945)	(1,180)	(2,365)	(4,381)
Ending balance	50,871	47,922	50,871	47,922
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	9,218	11,935	10,865	11,707
Provision for loan losses	(24)	557	(1,316)	1,485
Recoveries	192	119	476	868
Charge-offs	(425)	(739)	(1,064)	(2,188)
Ending balance	8,961	11,872	8,961	11,872
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,402	1,127	1,305	1,101
Provision for loan losses	229	157	317	211
Recoveries	3	17	12	25
Charge-offs				(36)
Ending balance	1,634	1,301	1,634	1,301
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	33,243	28,023	29,896	26,864
Provision for loan losses	1,091	424	4,325	2,556
Recoveries	85	50	345	141
Charge-offs	(54)	(29)	(201)	(1,093)
Ending balance	34,365	28,468	34,365	28,468
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,088	6,325	6,090	6,107
Provision for loan losses	154	277	539	838
Recoveries	135	91	382	400
Charge-offs	(466)	(412)	(1,100)	(1,064)
Ending balance	$ 5,911	$ 6,281	$ 5,911	$ 6,281