Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 4,326,636	$ 4,232,048
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 33,187	$ 17,605
Percent	0.77%	0.42%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 42,788	$ 33,575
Percent	0.99%	0.79%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 800,313	$ 795,803
Percent	18.50%	18.80%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 79,930	$ 87,304
Percent	1.85%	2.06%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 142,303	$ 150,620
Percent	3.29%	3.56%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 405,417	$ 403,664
Percent	9.37%	9.54%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 189,820	$ 184,707
Percent	4.39%	4.36%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 843,081	$ 821,251
Percent	19.48%	19.41%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 58,815	$ 65,477
Percent	1.36%	1.55%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,421,075	$ 1,356,430
Percent	32.84%	32.05%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 272,387	$ 283,636
Percent	6.29%	6.70%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 37,520	$ 31,976
Percent	0.87%	0.76%