Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis - Footnotes (Detail) - Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Footnotes to selected financial assets and liabilities:
Short-term equity securities held in trust	$ 12	$ 19
Long-term equity securities held in trust	$ 35	$ 42