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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 27,280 $ 21,109 $ 20,234
Mutual Fund | Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 11,888 9,905  
Mutual Fund | Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 15,392 11,204  
Fair Value, Inputs, Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 27,280 21,109  
Fair Value, Inputs, Level 1 | Mutual Fund | Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 11,888 9,905  
Fair Value, Inputs, Level 1 | Mutual Fund | Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 15,392 $ 11,204