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	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 39,320	$ 24,582	$ 26,408
Loan collectively evaluated for impairment	5,213,747	4,170,387	4,014,544
Total	5,253,067	4,194,969	4,040,952
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	4,618	3,093	8,189
Loan collectively evaluated for impairment	1,504,836	853,233	805,698
Total	1,509,454	856,326	813,887
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,056	1,376	1,047
Loan collectively evaluated for impairment	96,392	102,264	96,488
Total	97,448	103,640	97,535
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	33,170	19,787	16,787
Loan collectively evaluated for impairment	3,202,038	2,803,585	2,713,798
Total	3,235,208	2,823,372	2,730,585
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	476	326	385
Loan collectively evaluated for impairment	410,481	411,305	398,560
Total	$ 410,957	$ 411,631	$ 398,945