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%