Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis - Footnotes (Details) - Recurring [Member] - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Short-term equity securities held in trust | $ 11 | $ 19 |
| Long-term equity securities held in trust | $ 29 | $ 42 |