Summary of Significant Accounting Policies - Summary of Impacts of Revisions to the Risk-Free Discount Rate Applied to Environmental Remediation Liabilities and Recovery Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Charge (reduction) to operating expenses	$ (2)	$ (2)	$ 10
Risk-free discount rate applied to environmental remediation liabilities and recovery assets	2.50%	2.25%	2.00%