Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 4,390,775	$ 4,400,232
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 32,597	$ 33,793
Percent	0.74%	0.77%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 48,313	$ 47,283
Percent	1.10%	1.07%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 834,561	$ 828,260
Percent	19.01%	18.82%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 84,976	$ 84,228
Percent	1.93%	1.91%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 139,542	$ 144,751
Percent	3.18%	3.29%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 417,148	$ 420,884
Percent	9.50%	9.57%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 201,450	$ 197,872
Percent	4.59%	4.50%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 843,731	$ 846,360
Percent	19.22%	19.24%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 48,302	$ 57,806
Percent	1.10%	1.31%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,433,707	$ 1,426,643
Percent	32.65%	32.42%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 272,637	$ 279,704
Percent	6.21%	6.36%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 33,811	$ 32,648
Percent	0.77%	0.74%