Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 4,307,827	$ 4,232,048
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 39,160	$ 17,605
Percent	0.91%	0.42%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 39,930	$ 33,575
Percent	0.93%	0.79%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 792,335	$ 795,803
Percent	18.39%	18.80%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 75,627	$ 87,304
Percent	1.76%	2.06%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 136,150	$ 150,620
Percent	3.16%	3.56%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 402,901	$ 403,664
Percent	9.35%	9.54%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 191,218	$ 184,707
Percent	4.44%	4.36%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 845,360	$ 821,251
Percent	19.62%	19.41%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 59,967	$ 65,477
Percent	1.39%	1.55%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,410,730	$ 1,356,430
Percent	32.75%	32.05%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 277,671	$ 283,636
Percent	6.45%	6.70%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 36,778	$ 31,976
Percent	0.85%	0.76%