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Assets Measured at Fair Value on Recurring Basis for PBOP Plan (Detail) - Other Postretirement Benefit Plans, Defined Benefit - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 51,892 $ 44,770 $ 42,651
Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 23,025 20,156  
Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 28,867 24,614  
Fair Value, Inputs, Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 51,892 44,770  
Fair Value, Inputs, Level 1 | Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 23,025 20,156  
Fair Value, Inputs, Level 1 | Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 28,867 $ 24,614