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Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands | Mar. 31, 2026 | Dec. 31, 2025
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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
Utilities [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 1,857,775 | 1,810,095
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
Credit Rating, AAA [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 47,376 | 46,933
Credit Rating, AAA [Member] | Utilities [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 827,336 | 899,088
Credit Rating, AAA [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 874,712 | 946,021
Credit Rating, AA [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 51,712 | 51,390
Credit Rating, AA [Member] | Utilities [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 902,260 | 777,880
Credit Rating, AA [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 953,972 | 829,270
Credit Rating, A [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 372,892 | 379,973
Credit Rating, A [Member] | Utilities [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 110,407 | 114,845
Credit Rating, A [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 483,299 | 494,818
Credit Rating, BBB [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 804,924 | 812,061
Credit Rating, BBB [Member] | Utilities [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 15,316 | 15,824
Credit Rating, BBB [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 820,240 | 827,885
Credit Rating, BB [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 30,295 | 34,105
Credit Rating, BB [Member] | Utilities [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 2,456 | 2,458
Credit Rating, BB [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 32,751 | 36,563
Credit Rating, B [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 23,100 | 23,326
Credit Rating, B [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 23,100 | 23,326
Credit Rating, CCC-C [Member] | Competitive [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | 13,604 | 14,424
Credit Rating, CCC-C [Member] | State and political subdivisions [Member] |  |
Debt Securities Held To Maturity Credit Quality Indicator [Line Items] |  |
Held to maturity securities | $ 13,604 | $ 14,424

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