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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, 2015	Dec. 31, 2014
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis	$ 14,174	$ 12,840
Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis	6,620	5,661
Index Funds
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis		5,313
Equity Funds
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis	7,554	1,866
Fair Value, Inputs, Level 1
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis	14,174	12,840
Fair Value, Inputs, Level 1 | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis	6,620	5,661
Fair Value, Inputs, Level 1 | Index Funds
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis		5,313
Fair Value, Inputs, Level 1 | Equity Funds
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Assets measured at fair value on recurring basis	$ 7,554	$ 1,866

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- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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No definition available.

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- Definition

Schedule of Pension and Other Postretirement Plan Assets by Fair Value [Line Items]

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