Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 4,400,232	$ 4,232,048
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 33,793	$ 17,605
Percent	0.77%	0.42%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 47,283	$ 33,575
Percent	1.07%	0.79%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 828,260	$ 795,803
Percent	18.82%	18.80%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 84,228	$ 87,304
Percent	1.91%	2.06%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 144,751	$ 150,620
Percent	3.29%	3.56%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 420,884	$ 403,664
Percent	9.57%	9.54%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 197,872	$ 184,707
Percent	4.50%	4.36%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 846,360	$ 821,251
Percent	19.24%	19.41%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 57,806	$ 65,477
Percent	1.31%	1.55%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,426,643	$ 1,356,430
Percent	32.42%	32.05%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 279,704	$ 283,636
Percent	6.36%	6.70%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 32,648	$ 31,976
Percent	0.74%	0.76%