Loans Held for Investment and Allowance for Credit 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	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 39,226	$ 24,582	$ 28,508
Loan collectively evaluated for impairment	4,600,163	4,170,387	3,960,652
Total	4,639,389	4,194,969	3,989,160
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	5,620	3,093	8,897
Loan collectively evaluated for impairment	863,830	853,233	817,989
Total	869,450	856,326	826,886
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,128	1,376	831
Loan collectively evaluated for impairment	98,454	102,264	90,505
Total	99,582	103,640	91,336
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	32,156	19,787	18,254
Loan collectively evaluated for impairment	3,217,093	2,803,585	2,665,953
Total	3,249,249	2,823,372	2,684,207
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	322	326	526
Loan collectively evaluated for impairment	420,786	411,305	386,205
Total	$ 421,108	$ 411,631	$ 386,731