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	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 27,534	$ 17,670
Loan collectively evaluated for impairment	3,926,102	3,467,899
Total	3,953,636	3,485,569
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	9,334	3,612
Loan collectively evaluated for impairment	835,619	680,487
Total	844,953	684,099
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	759	134
Loan collectively evaluated for impairment	95,918	94,409
Total	96,677	94,543
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	16,714	12,838
Loan collectively evaluated for impairment	2,622,632	2,290,160
Total	2,639,346	2,302,998
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	727	1,086
Loan collectively evaluated for impairment	371,933	402,843
Total	$ 372,660	$ 403,929