Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 5,094,403	$ 4,975,976
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 74,596	$ 76,211
Percent	1.46%	1.53%
Tax-exempt [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 51,114	$ 48,415
Percent	1.00%	0.97%
Commercial and Financial [Member] | Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,062,527	$ 1,032,787
Percent	20.86%	20.76%
Commercial and Financial [Member] | Oil & Gas Production And Equipment [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 122,268	$ 94,729
Percent	2.40%	1.90%
Commercial and Financial [Member] | Agriculture [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 130,957	$ 136,313
Percent	2.57%	2.74%
Real Estate [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 450,525	$ 451,224
Percent	8.84%	9.07%
Real Estate [Member] | Farmland [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 227,635	$ 219,241
Percent	4.47%	4.41%
Real Estate [Member] | One to Four Family Residences [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 990,377	$ 979,170
Percent	19.44%	19.68%
Real Estate [Member] | Multifamily Residential Properties [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 68,743	$ 65,949
Percent	1.35%	1.33%
Real Estate [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1,525,064	$ 1,506,937
Percent	29.94%	30.28%
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 345,783	$ 328,069
Percent	6.79%	6.59%
Other (Not Classified Above) [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 44,814	$ 36,931
Percent	0.88%	0.74%