Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 4,984,484	$ 4,721,995
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 70,932	$ 73,827
Percent	1.42%	1.56%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 51,765	$ 48,626
Percent	1.04%	1.03%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,045,706	$ 995,207
Percent	20.98%	21.08%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 91,051	$ 95,574
Percent	1.83%	2.02%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 142,146	$ 141,249
Percent	2.85%	2.99%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 445,744	$ 437,277
Percent	8.94%	9.26%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 220,882	$ 195,162
Percent	4.43%	4.13%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 966,006	$ 875,766
Percent	19.38%	18.55%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 59,469	$ 46,030
Percent	1.19%	0.98%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,555,340	$ 1,487,927
Percent	31.21%	31.51%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 295,148	$ 284,373
Percent	5.92%	6.02%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 40,295	$ 40,977
Percent	0.81%	0.87%