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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 |

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