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Securities - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2012
Schedule Of Marketable Securities [Line Items] |  |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas | 99.70% |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 69.30% |  |
Carry Value of Securities Pledged | $ 3,600,000 | $ 3,800,000 |
Available for Sale, Estimated Fair Value | 12,420,446 | $ 11,387,321 |
Available for sale securities reclassified to held to maturity, unrealized gain |  |  | $ 165,700
Available for sale securities reclassified to held to maturity, unrealized gain net of tax |  |  | 107,700
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax | 1,800 |  |
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | $ 1,400 |  |
Reclassified To Held To Maturity [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Available for Sale, Estimated Fair Value |  |  | $ 2,300,000

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