Interest-bearing Time Deposits in Banks and Securities - Additional Information (Detail) $ in Thousands		12 Months Ended
	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Interest Bearing Time Deposits In Banks And Securities [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	$ 1,707	$ 1,707	$ 3,495
Held-to-maturity securities	$ 121	$ 121	278
Number of investment positions | Investment	466	466
Securities pledged as collateral	$ 1,940,460	$ 1,940,460	1,759,570
Sales of investment securities available-for-sale		40,510	35,580	$ 1,619
Gross realized gains from security sales		1,579	443	2
Gross realized losses from security sales or calls		$ 309	$ 11	$ 6
Obligations of State 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	33.91%
Obligations of State 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	82.13%
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