Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 4,721,995	$ 4,400,232
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 73,827	$ 33,793
Percent	1.56%	0.77%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 48,626	$ 47,283
Percent	1.03%	1.07%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 995,207	$ 828,260
Percent	21.08%	18.82%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 95,574	$ 84,228
Percent	2.02%	1.91%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 141,249	$ 144,751
Percent	2.99%	3.29%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 437,277	$ 420,884
Percent	9.26%	9.57%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 195,162	$ 197,872
Percent	4.13%	4.50%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 875,766	$ 846,360
Percent	18.55%	19.24%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 46,030	$ 57,806
Percent	0.98%	1.31%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,487,927	$ 1,426,643
Percent	31.51%	32.42%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 284,373	$ 279,704
Percent	6.02%	6.36%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 40,977	$ 32,648
Percent	0.87%	0.74%