Securities - Schedule of Aging of Past Due Held to Maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	$ 5,549,590	$ 5,691,868
Competitive [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	1,145,276	1,162,505
Competitive [Member] | 30-89 Days Past Due and Accruing [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	40,570
Competitive [Member] | Total Past Due [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	40,570
Competitive [Member] | Current [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	1,104,706
Utilities [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	1,655,383	1,667,900
Utilities [Member] | Current [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	1,655,383
State and political subdivisions [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	2,800,659	$ 2,830,405
State and political subdivisions [Member] | 30-89 Days Past Due and Accruing [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	40,570
State and political subdivisions [Member] | Total Past Due [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	40,570
State and political subdivisions [Member] | Current [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Total held to maturity securities	$ 2,760,089