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Weighted Average Asset Allocations for Retirement Plan (Detail) | Dec. 31, 2017 | Dec. 31, 2016
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Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocation | 100.00% | 100.00%
Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocation | 59.00% | 56.00%
Debt Securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocation | 39.00% | 43.00%
Cash Equivalents |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocation | 2.00% | 1.00%

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