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Actual Investment Allocations (Detail) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 100.00% | 100.00% | 100.00%
Pension Plans | Equity Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 46.00% | 46.00% | 49.00%
Pension Plans | Fixed Income Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 37.00% | 37.00% | 36.00%
Pension Plans | Equity And Debt Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation | [1] |  | 7.00% | 6.00% | 5.00%
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 100.00% | 100.00% | 100.00%
Other Postretirement Benefit Plans, Defined Benefit | Equity Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 55.00% | 53.00% | 56.00%
Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 43.00% | 47.00% | 44.00%
Other Postretirement Benefit Plans, Defined Benefit | Other |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation | [2] |  | 2.00% | 0.00% | 0.00%
Scenario Forecast | Pension Plans | Equity Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation |  | 47.00% |  |  |
Scenario Forecast | Pension Plans | Fixed Income Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation |  | 37.00% |  |  |
Scenario Forecast | Pension Plans | Equity And Debt Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation | [1] | 6.00% |  |  |
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Equity Funds |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation |  | 55.00% |  |  |
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation |  | 45.00% |  |  |
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Other |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation | [2] | 0.00% |  |  |
Real Estate Funds | Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 10.00% | 11.00% | 10.00%
Real Estate Funds | Scenario Forecast | Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target Allocation |  | 10.00% |  |  |
[1] | Represents investments in an asset allocation fund. This fund invests in both equity and debt securities.
[2] | Represents investments being held in cash equivalents as of December 31, 2016 pending transfer into debt and equity funds.

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