Securities - Additional Information (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2021 USD ($) Investment	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) Investment	Sep. 30, 2020 USD ($)
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years			12 months
Number of investment positions \| Investment	181		181
Securities pledged as collateral	$ 3,262,605,000		$ 3,262,605,000
Sales of investment securities available-for-sale		$ 10,084,000	10,631,000	$ 263,042,000
Gross realized gains from security sales	1,000	36,000	814,000	3,614,000
Gross realized losses from security sales or calls	$ 0	$ 0	$ 0	$ 4,000
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	54.16%		54.16%
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			74.96%

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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- Definition

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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- Definition

Period of time between issuance and maturity of customer deposits, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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