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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | $ 20,234 | $ 16,606 | $ 14,174
Cash and Cash Equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 400 | 400 |
Mutual Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 19,834 | 16,206 |
Mutual Fund | Fixed Income Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 8,419 | 7,078 |
Mutual Fund | Equity Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 11,415 | 9,128 |
Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 20,234 | 16,606 |
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 400 | 400 |
Fair Value, Inputs, Level 1 | Mutual Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 19,834 | 16,206 |
Fair Value, Inputs, Level 1 | Mutual Fund | Fixed Income Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | 8,419 | 7,078 |
Fair Value, Inputs, Level 1 | Mutual Fund | Equity Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets measured at fair value | $ 11,415 | $ 9,128 |

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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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