Loans Held for Investment and Allowance for Loan Losses - Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 25,587	$ 17,670	$ 18,750
Loans collectively evaluated for impairment	3,841,453	3,467,899	3,453,477
Total	3,867,040	3,485,569	3,472,227
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	6,961	3,612	4,133
Loans collectively evaluated for impairment	766,963	680,487	670,814
Total	773,924	684,099	674,947
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,046	134	60
Loans collectively evaluated for impairment	92,907	94,409	82,945
Total	93,953	94,543	83,005
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	16,682	12,838	13,386
Loans collectively evaluated for impairment	2,598,247	2,290,160	2,284,170
Total	2,614,929	2,302,998	2,297,556
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	898	1,086	1,171
Loans collectively evaluated for impairment	383,336	402,843	415,548
Total	$ 384,234	$ 403,929	$ 416,719