Securities - Additional Information (Detail) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020 USD ($) Investment	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Investment	Jun. 30, 2019 USD ($)
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Number of investment positions \| Investment	14		14
Securities pledged as collateral	$ 2,685,828		$ 2,685,828
Sales of investment securities available-for-sale	157,521	$ 65,821	252,958	$ 66,052
Gross realized gains from security sales	1,516,000	689,000	3,578,000	693,000
Gross realized losses from security sales or calls	$ 4,000	$ 13,000	$ 4,000	$ 17,000
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	51.14%		51.14%
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			88.76%

Interest Bearing Time Deposits In Banks And Securities [Line Items]

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Percentage of investment portfolio guaranteed by permanent school fund.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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