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Securities - Shedule of Aging of Past due Held to Maturity Securities (Detail) - USD ($) $ in Thousands | Mar. 31, 2026 | Dec. 31, 2025
--- | --- | ---
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | $ 5,703,238 | $ 5,724,227
Competitive [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 1,343,903 | 1,362,212
Competitive [Member] | 30-89 Days Past Due and Accruing [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 131 |
Competitive [Member] | Greater than 90 days Past Due and Accruing [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 16,470 |
Competitive [Member] | Total Past Due [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 16,601 |
Competitive [Member] | Current [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 1,327,302 |
Utilities [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 1,857,775 | 1,810,095
Utilities [Member] | Current [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 1,857,775 |
State and political subdivisions [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 3,201,678 | $ 3,172,307
State and political subdivisions [Member] | 30-89 Days Past Due and Accruing [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 131 |
State and political subdivisions [Member] | Greater than 90 days Past Due and Accruing [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 16,470 |
State and political subdivisions [Member] | Total Past Due [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 16,601 |
State and political subdivisions [Member] | Current [Member] |  |
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] |  |
Held to maturity securities | $ 3,185,077 |

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