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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 21,109 $ 20,234 $ 16,606
Mutual Fund      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value   19,834  
Mutual Fund | Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 9,905 8,419  
Mutual Fund | Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 11,204 11,415  
Cash and Cash Equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value   400  
Fair Value, Inputs, Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 21,109 20,234  
Fair Value, Inputs, Level 1 | Mutual Fund      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value   19,834  
Fair Value, Inputs, Level 1 | Mutual Fund | Fixed Income Securities      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value 9,905 8,419  
Fair Value, Inputs, Level 1 | Mutual Fund | Equity Funds      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value $ 11,204 11,415  
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets measured at fair value   $ 400