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Actual Investment Allocations (Detail) | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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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 |  |  | 49.00% | 46.00% | 46.00%
Pension Plans | Fixed Income Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 34.00% | 37.00% | 37.00%
Pension Plans | Equity And Debt Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation | [1] |  | 6.00% | 7.00% | 6.00%
Pension Plans | Other |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation | [2] |  | 1.00% |  |
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 |  |  | 56.00% | 55.00% | 53.00%
Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 42.00% | 43.00% | 47.00%
Other Postretirement Benefit Plans, Defined Benefit | Other |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation | [3] |  | 2.00% | 2.00% | 0.00%
Real Estate Funds | Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Actual Allocation |  |  | 10.00% | 10.00% | 11.00%
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 | [3] | 0.00% |  |  |
Scenario Forecast | Real Estate Funds | 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, 2017 pending payment of benefits.
[3] | Represents investments being held in cash equivalents as of December 31, 2017 and 2016 pending transfer into debt and equity funds.

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