Commitments and Contingencies - Range of Environmental Liability and Number of Active Sites for Environmental Matters (Detail) - Environmental Related Matters [Member] $ in Millions	Dec. 31, 2017 USD ($) site	Dec. 31, 2016 USD ($) site
Environmental Matters Range Of Estimated Liability And Active Sites Numbers [Line Items]
Number of active environmental investigation and remediation sites | site	36	39
Maximum [Member]
Environmental Matters Range Of Estimated Liability And Active Sites Numbers [Line Items]
Possible High End Range of Environmental Liability | $	$ 126.3	$ 127.6