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Retirement Benefit Plans - Assets Measured at Fair Value on Recurring Basis for PBOP Plan (Detail) - Retirement Benefits - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 58,402 $ 51,892 $ 44,770
Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 25,908 23,025  
Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 32,494 28,867  
Fair Value, Inputs, Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 58,402 51,892  
Fair Value, Inputs, Level 1 | Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 25,908 23,025  
Fair Value, Inputs, Level 1 | Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 32,494 $ 28,867