Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 5,626,437	$ 5,691,868
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,147,720	1,162,505
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,664,344	1,667,900
State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	2,812,064	2,830,405
Credit Rating, AAA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	604,178	757,381
Credit Rating, AAA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	604,178	757,381
Credit Rating, AA [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	8,723	7,704
Credit Rating, AA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	929,491	795,448
Credit Rating, AA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	938,214	803,152
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	452,264	464,349
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	102,810	87,736
Credit Rating, A [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	555,074	552,085
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	615,241	641,743
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	27,464	26,720
Credit Rating, BBB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	642,705	668,463
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	34,079	30,734
Credit Rating, BB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	401	615
Credit Rating, BB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	34,480	31,349
Credit Rating, B [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	14,230	15,326
Credit Rating, B [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	14,230	15,326
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	23,183	2,649
Credit Rating, CCC-C [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 23,183	$ 2,649