Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis - Footnotes (Detail) - Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Footnotes to selected financial assets and liabilities:
Short-term equity securities held in trust	$ 11	$ 11
Long-term equity securities held in trust	$ 24	$ 29