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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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