v3.26.1
Securities - Schedule of Aging of Past due Held to Maturity Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities $ 5,716,426 $ 5,724,227
Competitive [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 1,338,149 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 8,276  
Competitive [Member] | Total Past Due [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 8,276  
Competitive [Member] | Current [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 1,329,873  
Utilities [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 1,934,032 1,810,095
Utilities [Member] | Current [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 1,934,032  
State and political subdivisions [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 3,272,181 $ 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 8,276  
State and political subdivisions [Member] | Total Past Due [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities 8,276  
State and political subdivisions [Member] | Current [Member]    
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]    
Held to maturity securities $ 3,263,905