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, 2019	Dec. 31, 2018	Jun. 30, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 26,408	$ 27,534	$ 26,685
Loan collectively evaluated for impairment	4,014,544	3,926,102	3,804,795
Total	4,040,952	3,953,636	3,831,480
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	8,189	9,334	7,580
Loan collectively evaluated for impairment	805,698	835,619	767,430
Total	813,887	844,953	775,010
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,047	759	1,259
Loan collectively evaluated for impairment	96,488	95,918	91,324
Total	97,535	96,677	92,583
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	16,787	16,714	16,715
Loan collectively evaluated for impairment	2,713,798	2,622,632	2,551,163
Total	2,730,585	2,639,346	2,567,878
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	385	727	1,131
Loan collectively evaluated for impairment	398,560	371,933	394,878
Total	$ 398,945	$ 372,660	$ 396,009