Pension and Other Postretirement Benefits - Asset Categories Based on the Nature and Risks of the Plan Assets (Parenthetical) (Detail) - Defined Benefit Pension Plan [Member] - U.S. Defined Benefit Pension Plans [Member] - Managed Funds [Member] |
Dec. 31, 2017 |
Dec. 31, 2016 |
---|---|---|
Equity Securities [Member] | ||
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | ||
Target Allocation | 47.30% | 45.90% |
Alternative Investments [Member] | ||
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | ||
Target Allocation | 41.10% | 44.50% |
Fixed Income Securities [Member] | ||
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | ||
Target Allocation | 9.60% | 7.40% |
Other, Primarily Cash And Cash Equivalents [Member] | ||
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | ||
Target Allocation | 2.00% | 2.20% |