Bank Premises and Equipment - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 10,130,000	$ 9,810,000	$ 9,390,000
Rental income for leases included in net occupancy expense	2,682,000	2,367,000	2,139,000
Gain (loss) on sale the of bank premises and equipment	(147,000)	(396,000)	168,000
Gain (loss) on sale of building	$ 1,216,000	1,167,000	1,894,000
Weatherford [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Building			1,385,000
Gain (loss) on sale of building			560,000
Orange [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Building			2,000,000
Gain (loss) on sale of building			$ (31,000)
San Angelo [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Building		1,586,000
Gain (loss) on sale of building		210,000
Write off of leasehold improvements		$ 360,000