Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 5,549,590	$ 5,691,868
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,145,276	1,162,505
State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	2,800,659	2,830,405
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,655,383	1,667,900
Credit Rating, AAA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,577,195	757,381
Credit Rating, AAA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,577,195	757,381
Credit Rating, AA [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	9,080	7,704
Credit Rating, AA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	9,080	803,152
Credit Rating, AA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities		795,448
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	442,509	464,349
Credit Rating, A [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	495,297	552,085
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	52,788	87,736
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	641,441	641,743
Credit Rating, BBB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	666,841	668,463
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	25,400	26,720
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	30,112	30,734
Credit Rating, BB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	30,112	31,349
Credit Rating, BB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities		615
Credit Rating, B [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities		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		15,326
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	22,134	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	$ 22,134	$ 2,649