Assets Measured at Fair Value on Recurring Basis for PBOP Plan (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $)
In Thousands, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2011

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

Assets measured at fair value on recurring basis

$ 8,301

$ 7,339

Fixed Income Securities

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

Assets measured at fair value on recurring basis

3,670

3,311

Index Funds

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

Assets measured at fair value on recurring basis

3,357

2,977

Equity Funds

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

Assets measured at fair value on recurring basis

1,274

1,051

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

8,301

7,339

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

3,670

3,311

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

3,357

2,977

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

1,274

1,051

Fair Value, Inputs, Level 2

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 2 | Fixed Income Securities

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 2 | Index Funds

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 2 | Equity Funds

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 3

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 3 | Fixed Income Securities

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 3 | Index Funds

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

Assets measured at fair value on recurring basis

Fair Value, Inputs, Level 3 | Equity Funds

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

Assets measured at fair value on recurring basis