Interest-bearing Time Deposits in Banks and Securities - Additional Information (Detail)		3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) Investment	Sep. 30, 2016 USD ($) Investment	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Investment	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Interest Bearing Time Deposits In Banks And Securities [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	$ 1,707,000	$ 1,707,000	$ 4,491,000	$ 1,707,000	$ 4,491,000	$ 3,495,000
Held-to-maturity securities	$ 129,000	$ 129,000	286,000	$ 129,000	286,000	$ 278,000
Number of investment positions | Investment	89	89		89
Securities pledged as collateral	$ 1,805,813,000	$ 1,805,813,000		$ 1,805,813,000
Sales of investment securities available-for-sale		7,410,000	27,910,000	20,792,000	34,541,000
Gross realized gains from security sales		239,000	142,000	1,158,000	390,000
Gross realized losses from security sales or calls		$ 0	$ 6,000	$ 5,000	$ 10,000
Obligations of States and Political Subdivisions [Member] | Texas [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Percentage of securities guaranteed by Texas Permanent School Fund	32.00%
Obligations of States and Political Subdivisions [Member] | Texas [Member] | Available-for-Sale Securities [Member] | Geographic Concentration Risk [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Concentration risk, percentage	81.51%
Minimum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years				1 year
Maximum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years				3 years